I read lots of reviews on lots of sites before trying these bottles and was sceptical but I have found them to be fantastic.
You wouldn't want to try to mix formula in them as I think it would be hard to get the water amount in and be confident it was right, but I think they are great for using with ready to use formula. They are packaged individually so if you need to pop one bottle in your bag you can and its small. They are strange to hold at first, with them just being a bag, but the knack is to hold by the neck and then they are fine.
One other thing, if your child is a difficult feeder, i.e. bottle in and out, then maybe they aren't for you.
All in all they are great for travelling and occasional use.