I have just ordered a new one of these after my 3yr old one has finally given up the ghost - the reason being that I have often put boiling hot bottles in the bag (which it says you shouldn't) and therefore the lining has come away on the inside. However if the bottles are just off boiling, this shouldn't happen.
It is true (as other reviwer's say) that it certainly does not keep at the same temperature over 4hrs long. However I have found this with all the other thermal bags I have tried. Essentially you just need to add a gel pack to keep cool, or make the water hotter (depending how long it needs to stay warm) and just take milk powder in a capsule alongside. We've often done this for night feeds in the winter, so that come 3am in the morning the water is still warm-ish and therefore acceptable for baby. Also on long journeys this has worked well too.
I also use the tommee tippee thermal bags which are for individual bottles, these don't hold warmer longer but are handy if you just have one bottle with you, when e.g. you are out shopping. OR as we have found putting the bottle in the tommee tippee and then squashing into the avent therma bag keeps it pretty hot all night!
A bonus is the pocket on the outside, which means you can put some hand gel and a dummy or two in if you need to.
All in all, I would definitely recommend this, but you need to be forewarned / realistic about thermal bag performances in general.