I don't understand why they don't have a web app, especially when competitors like Last.fm, Grooveshark, Turntable.fm and others do.
P2P is all about lowering their costs, not about making better sound quality for low connection speeds. It doesn't matter if you're streaming from Spotify's data center or across the street, slow is as slow does. You're not squeezing more bits through a slow connection because of P2P.
I don't know if I expressed myself any more clearly :)
Other than that, its quite responsive.