Wow, this filter is awesome! I've been in this hobby for a while now, and this is one of the best filters I've used. My first canister was a Fluval 306, which was great for my 40 gallon. Once the Fluval 306 began to fail after 3.5 years, I replaced it with a Sunsun 304b. While using the Sunsun, I grew to love stack on media trays due to the ease of maintenance and the large amount of room for my mechanical, chemical, and biological media. I purchased a 125 gallon aquarium recently and thought about purchasing two Fluval FX6 canisters, but didn't like the way the inside of the canister was setup (personal preference). So, I took a chance with two Eheim Pro 4+ 600's, and couldn't be happier!

Pros
- incredibly quiet
- light weight
- transparent canister siding (helped me gauge the amount of tank water I let into the canister to keep my biological media wet, while continuing the filter install)
- motor head is easy to put on and take off canister (this makes filter maintenance a breeze)
- comes with pretty nice biological media (although I use Bio Home Ultimate)
- energy efficient.

Cons
- spray bar isn't easily adjustable
- could be slightly larger

Only reason I am giving this filter 4 stars is because I just got it going yesterday. I bought this to replace a fluval G6 that started to have problems after only 5 months.

So far I have to say I really like everything about this filer.

1. Very quiet operation. I actually have mine outside my cabinet and really don't even hear it.

2. Lots of media. I really like how much media fits into this filter. 2 sponges, 2 bio trays and 2 mech trays are really nice. Most likely I'll switch them all to bio eventually but gives you lots of room for options.

3. This filter moves some serious water. The G6 I had running on my planted discus tank does not even compare in flow. The G6 was barely enough while I had to throttle the eheim back to nearly 50%. This only being because I have discus. For anyone with a heavily planted tank or fish that enjoy a lot of flow this baby is for you.

4. Filter seems that it will be very easy to disconnect to clean. Haven't gotten that far yet but will update this review when I do

I'd definitely recommend this filter to anyone searching for a powerful filter with lots of media.

I have owned many many fish filters and now with a 60 gallon tank and huge parrot and catfish, I have to say, it is easy to clean, and quiet (sits on the counter in my kitchen!) It does a great job.

I've been using the Eheim Pro 4+ 600 for about a year now on my 125 gallon tank. This filter works in conjunction with a fluval fx6. The Eheim 4+ serves as the primary bio filtration unit. With the included media, you get a good starter pack of material and only need to add new material later down the road. The only issue one might find with this unit is the included hose. I needed to order more eheim hose to get both the inlet and outlet the right length. Fortunately i read the reviews and ordered it at the same time, which saved me a lot of delay.
With the hose issue out of the way, this is a super easy canister to assemble and start. Once the media is rinsed, one simply puts it in the internal trays and fills the canister with water. Once the pump unit is secured, the hose fitting snaps on easily. A few pushes of the priming pump floods the remainder of the unit and it's ready to turn on. No need to manually prime and get aquarium water in my mouth. That's my one complaint with priming my eheim 1500xl (other tank).
Maintenance is much easier than the fx6 because of how easy it is to remove the hose fitting. Just push a switch over and the water is held in the hoses until you're ready to reattach. The push button release works without having to fight with the pressure of the canister. For the first cleaning, I rinsed the blue sponge, tossed the floss media layer, and rinsed the stone and plastic bio material. After a year of operation, the 4+ still pushes water with the same steady flow. I highly recommend the separately purchased eheim pre filter to greatly reduce maintenance. I use them on both this unit and fx6.
The tank size rating for the 4+ 600 is debatable in my opinion. With a few small to medium fish, you could possibly get away with just this unit alone on a 125-150 gallon aquarium. However, with a full community of medium sized fish, or a few large fish, I highly recommend another unit to help the 4+. It would take at least two 4+ 600 units to get the media volume of the eheim 1500XL (2262) I use on my 180 gallon. Even with two units the gph would still lag behind that beast of a canister. However, the 4+ 600 is hard to beat at half the price or less. The water gets good contact time with the media. When combined with the fx6, they make a very efficient pairing of bio and mechanical focused filtration.

On the one hand, it doesn't have nearly the flow of my Fluval FX4 9or of my old Eheim 3), and this filter is a royal pain to prime. On the other hand, Eheims are so finely crafted that you only need to clean it once or twice a year. It's getting the job done and that's more than half the battle.