This is my third noise-cancelling headphones and expectations were high, especially for a Sennheiser product. But the noise cancellation is very weak and does not stand in comparison to other headphones.
There are many controls that are not labeled in any way and for a while, you have to take the phones off and compare the button layout with the manual.
Otherwise, they are quite comfortable and the sound quality is good.

Definitely good for the price point, but they get uncomfortable after extended use. I work in an office, so I have to wear them for zoom calls all day. The weight distribution puts a lot of pressure on the head band and tops of the muffs.

The battery life is really excellent as it lasted me four entire international flights and still had 80% left. I also love that they are not too big nor too small. Just the perfect size for the average human ear.
The app works flawlessly on my iPhone and the USB-C charging port is great because it is the standard port in 2023. I used these very cool 'C&D Visionary DC Comics Batman Logo' stickers I found here on Amazon to customize them and they ended up looking is if these were a 'Batman Special Edition' version. Super cool. The sound is crystal clear and far superior to my Apple AirPods. I cannot recommend these headphones enough.

The sennheiser HD450 BT is the successor to the 4.50. it's an improvement in every way.
Comfort.
The 450 BT has a thin headband but it never dug into my head at all. This is all plastic but it's well constructed and folds to a very compact size. The one thing i wish is that the earcups were a smidge wider, but i can think of a number of people i know and who i had try these headphones on, of varrying builds and none of them felt comfort was an issue. Truthfully it isn't anything i can't live with and i am nitpicking, but also comparing to other headphones i own.
The headphones charge via USB-C and also have an analog cable with a 2.5 MM plug for the headphones, and a 3.5 MM for your headphone jack.
The codecs they support are SBC, AAC, APTX and APTX-LL. The latter is designed to sync the audio from a TV or movie with what you see on screen.
Note; IPHONES AND IPADS DO NOT SUPPORT THIS FEATURE.
noise canceling. The headphones do well illiminating the whirring from AC units, busses etc, but with Air Conditioners for example, you'll still hear the fan noise, though it will be reduced. These also don't block voices very well, though they passively block them out enough so that with music playing at low-medium volumes and higher of course, you likely won't hear any of these noises well. There are headphones that cost double who's ANC is not as good as the Sennheiser HD450 BT.
Sound. You can customize the sound via the Sennheiser Smart Control app, available on IOS and Android, via a dot EQ. The eq implementation is 2 stars only because i am able to get a sound i enjoy with these and other Sennheiser products i have which also use the Smart Control app, but from a programming and customization standpoint, it earns a solid half a star.
By default the sound has an emphasis on bass in all it's forms, but these are not muddy by any means. The mids are full and the treble is rolled off slightly, but fortunately all this can be refined to some degree with the eq. Theres still a veil to the sound if i compare them to higher priced models like the NAD Viso HP70 or Shure Aonic 50, but those cost twice as much as the $200 MSRP Sennheiser placed on these. that said, these have some real potential and if you use an app that has a multiband eq, you could really make these shine. AS is, they have a good sense of space for instruments to have separation and sounds in tv shows to have their place as well.
Functionality.
The Sennheiser 450 BT uses buttons for it's functions.
on the underside of the left side of the headphone just above the earcup, there is a braille L to indicate left. The right earcup is where all the buttons and ports are.
you have the analog adapter mentioned earlier, the USB-C port, on the bottom edge of the right earcup. The volume button has a dot on one end, that's volume up. the other side closest to you while wearing them is volume down. Details like this help when you want to use them without looking, especially because while the buttons aren't hard to use, they are a little cramped in some instances. Fortunately they're distinct enough that it shouldn't be hard memorizing each one and what it does. Back to the layout, the next button over from volume moving towards your ear while you're wearing them is a toggle which rewinds and fastforwards by flicking it it up for fast forward, and down for rewind. Fun fact, if you hold the toggle in either direction, you can scrub through the track bit by bit. perfect if you miss something while listening to a podcast or audiobook. some devices and apps don't support this functionality however. pressing inward on the toggle pauses or plays music and also answers and ends calls. next button up, right near where the headphone band connects to the earcup is an assistent button. This worked 100% of the time. I didn't need to set it up like i have to with some products. This is using google Assistent. on the opposite side of the right earcup from the assistent button is the power/noise cancelation button. You press and release to turn ANC on and off and hold for 2-3 seconds to power on and off. Hold for 5 seconds to place in pairing mode.
These also support multipoint connection, so you can have them connected to a tablet for music and a phone for calls for example. If you have music playing on one device, and recieve a call on another device the headphones are paired to, the music will pause, and the call will come in. you can hold the function button for a couple of seconds to reject the call. IF you do, your audio from the previous device will start playing again.
Call quality. While these headphones don't have a transparency mode available while listening to music etc, they do use it for phone calls. This is a wonderful feature because you can clearly hear yourself and your environment while on a call. Vital while you're walking or might otherwise be occupied. The callers i spoke with in most cases didn't know beforehand i was using headphones for the call. And they never ask me to repeat myself or if i were on bluetooth. while speaking with them i did every day tasks, washing dishes, talking with my birds, vacuuming etc. The people i did ask for feedback on how i sounded said they could hear these noises in the background, but the sounds didn't overpower my voice and they could always hear me clearly. I left my phone in a room most the time while i wandered through my apartment and it never cut out on the call or while listening to music.
These are a fantastic buy. If you want a well built headphone that has a lot of features but doesn't have a premium price, this is the one to get right now.

These are definitely a great choice for casual listeners in my opinion!

Some downsides I must say are that they are uncomfortable for long periods of time, especially if you wear glasses. Im definitely on the smaller side of sizing so it's not extremely noticeable for me unless I'm wearing my glasses or it's for hours at a time. The ANC is also rather odd. I'm itself it's fine, but sometimes you can catch a few odd things through it, and I wouldn't be able to use these without the ANC on. It has a weird sort of static without the ANC enabled, almost sounds like listening through a sea shell lol. Additionally, the buttons are initially pretty awkward and odd to get used to.

However, overall I do really enjoy these headphones. As someone who got these as an upgrade from some falling apart 10 year old sound leaking skull candy headphones this was a great choice. The ANC is well, and even just putting the headphones on and not playing anything I can see a difference. The app is fun if it's something you've never gotten a chance to use with something like this before. And the battery life is pretty solid. These are great for when I'm getting lost in some work for a bit. I love putting them on and hearing nothing around me, and being able to hear every sound in what I'm listening to through the duration of the song. It's truly a great feeling.

If you want something more for being out and about though, I may suggest something else. If I'm out and about or around other people, it can be difficult, from not hearing people talk to only being able to wear them for short periods of time.

They come with a charging cable (USB C port), case, and jack. Which I thought was pretty neat, it was all organized nicely and I throughly enjoy using the case.

Overall, I wish they were more comfortable, and the battery life was a bit longer. But I think after some time they are a good fit!