We have a west highland terrier puppy which we brought home at 8 weeks. We have used this bag every single day and both agree that it is the most useful, and best value, purchase of everything we've bought for her (about £500 worth of kit, toys etc).
We put the same bedding inside it that is in her bed so that she recognises it as a safe place, and for the first four weeks when she wasn't allowed to go on the floor (as she was waiting for her vaccinations) it was a godsend. Every day we were able to take her out in the car, on the bus, to cafes, to pubs, to people's houses - and the result is that she is calm, confident, well socialised puppy whilst still being safe and sound in her bag.
Great purchase and great price, cannot recommend enough.
p.s. a great tip is to get this and a small kong toy for a puppy - each time you go to a new place give her the kong filled with some treats to distract her in the bag while she gets used to the new noises and smells.