Tell me about Shelties - are they a good match for our family?

[Swisswatch]

My DD has just 'babysat' a Sheltie for a few days for a friend. She is utterly in love (as am I).
The dog seemed to be a really good match temperament-wise for us. She was a bit timid, needed exercise but not enormous amounts, was great on the lead and very affectionate.

We've been thinking of getting a dog recently, and the dog shelter locally only seems to be getting in either teeny-weeny dogs (DH would object) or great massive things (I'm not keen). I've read a bit about Shelties, and they seem to be fab, but are there any drawbacks, especially as they're purebred?

Thanks for any advice.