My 6 year old got 2 go go hamsters and on carpet they tend to get bits of fluff caught in the wheels. They run much better in the fun house and my daughter giggles her little head off watching them run round and around and bump into each other. I think more than two might make it a bit crowded and my daughter wouldn't find it as funny with just one go go hamster.
So far we have the sleep dome add on (which the hamsters get stuck in so we had to block it off) and the sk8t board and u turn which works well but you have to attach the board to the hamster yourself.
I've found the house stays together well and isn't at all flimsy. The house has notches on the floor that the hamster "reads" but we haven't figured out yet what that actually does! The tunnel between the "bedroom" and "kitchen" has a lid that lifts up because the hamsters do occasionally get stuck.
At £20 I dont think it's overpriced and my daughter has spent far more time playing with it than any of her other Christmas toys.
Overall, I think it could of been done a bit better but my 6 year old loves it and if she's happy, I'm happy. I'd recomend it if you have carpet and even if you dont, it keeps the hamsters in one place and is more fun than watching them on a big space.