I bought this to use with my 13 lb terrier.  It is a tight fit and she barely makes it and she looks uncomfortable.  She is used to a pet stroller and I thought this would be useful when a stroller is too difficult to transport.  I am still getting her used to it around the house.  It is more confining than a stroller maybe because she is on the larger size for a medium and she seems to want to jump out immediately.  I am glad to have it for emergency situations where a stroller is too big but for us it is definitely not a replacement.    

Pros: will be good for emergency situations when it is too much for me to carry 13 lbs!!    
Cons: the medium seems to small for this particular dog.  She can't move enough in it.