Second baby and still using the Kiinde Twist system. I LOVE that I can pump directly into the bags, but I even love the bags and bottles when I pump using my hands-free Freemie cups. Here is where I think the Kiinde system excels (even though arguably more expensive than other bags).
1. The nipple shape - My babes have been exclusively breastfed up until I return to work at 3mo. To be honest, the oblong shape of these nipples is FAR closer resemblance to my breast-shape :)
2. Slow flow - I never move my kiddos up off the slow flow until they are over 10mo. The slow flow of this bottle ensures they don't prefer the bottle over breast and I can continue to nurse during the evenings and overnight with no issues.
3. No air - the bag system allows the bag to collapse at baby drinks, which leads to far less gas and air intake.

Couple of warnings:
- The bags themselves are not accurate measures of ounces. They are close, but if you need them to be exact then you are going to measure in another bottle first.
- Definitely not the cheapest option, but the supplies hold up over years, so I was able to reuse everything for baby #2!

I am a huge fan of the Kiinde brand. I was skeptical at first since my son has gone through many different bottles and nipples which didn't work for him, however he took to this feeding system very easily. The system is very user friendly, as well as easy to clean. I don't produce enough breastmilk to actually use the system to freeze and serve using the container but its been nice to use for formula when we are out and about because you don't have to worry about carrying around dirty bottles. You can just throw away the used package and the nipples are easy to stow away in their cases. I would definitely recommend this product to new parents because its easy to use, very affordable, and there are quite a few companies that accept insurance that will send you Kiinde bags and starter kits with your breast pump. And the great thing is that when your LO is done using the bags for milk you can use them for pureed foods with their Foodii system!

i love the idea of the Kiinde system. its great for the frozen milk where all you have to do is thaw it and snap on the nipple then your ready to go.
pros:
you can pump directly into the bag. freeze bag. thaw bag
no bottles to wash (easy clean up)
super easy to use
cons:
this is on the expensive side
the volume of milk isn't clear no matter how you expand the bag. you can expand too far or not enough. because of this i still pour into a medela bottle to get accurate measurements before freezing.

even with the cons i am still using this system because the nipple flow on these is just right for my little man. it definitely helps with gas issues. i can tell a big difference with the flow of milk on this nipple vs other brands (medela; dr brown)
i recommend this for working moms still trying to provide breastmilk for their little ones.

If you want to save time washing, pumping, and putting bottles together then this is the product you need. I pump directly into the bags which I can them freeze or use in a bottle to give to daycare. This eliminates lost milk when transferring to bottle-bottle or bottle-storage bag. Since it useds bags it's much easier to warm as well. There are two bottle and two nipples. One is a slow flow nipple and the other is a regular flow nipple. It does not come with the plastic milk bags. My daughter is breastfed and takes these bottles easily.

I can't say enough about how much I LOVE this system!! The Twist pouches I can pump into, freeze, and feed! The adapter works with my Spectra S2 pump. I was disappointed to find out my daycare wouldn't allow me to send breastmilk in these bottles because the large shape of the bottle "sleeve" would not fit into their bottle warmer. I do use them at home because I can heat the pouches and then assemble the bottle (daycare required bottles to be ready to feed with no assembly required, which I understand). Freezing milk is so handy and these pouches are very durable and seal completely so I don't worry about leaking. I can write on the cap or front of the pouch. I heard complaints about inaccurate volumes but I simply pull the bottom of the pouch out once it begins to fill, as instructions recommend, or pour into a bottle to accurately measure. The one and only complaint I have would be that when transferring milk from pouches to bottles (I use Avent bottles for daycare) the shape of the pouch being kind of square at the top leaves some of the thicker "fat" residue in the pouch, which is important for baby to get. I have tried swirling around the pouch but some still sticks. The nipples are so great because I feel baby has to suck, mimicking nursing so not worried about confusing her. Pouches allow you to push air out of them when assembling bottles to reduce ingested air. Overall highly satisfied with this system.