I have used the Playtex drop-in liner bottle system for my son since he first began taking bottles. I used an Avent feeding system with my daughter and much prefer the Playtex.
These bottles have a rubber nipple which my son took to quite well. The bottle itself is hollow and the milk is placed into a liner that is dropped into the hole. The thing I love most about this system is that you can squeeze all the air out of the liner, which reduces the amount of air your baby swallows during feeding. It is also nice because you don't have to tilt the bottle up in the air in order for them to get liquid out- they can simply hold it straight up and down and suck out the fluid.
I also liked the drop-in liner system because the milk is a lot easier to heat up. My baby was bottle-fed breastmilk and it is difficult to reheat without ruining the milk. If you place a bottle with a liner in it in a cup of hot water, it heats up more quickly than a different type of bottle. Quick is a big plus when you have a crying baby to feed.
These are very easy to clean because the liners are disposable (although we have rinsed many out and re-used them). A drawback is that you do have to occasionally purchase the liners (unless you rinse them out to reuse).