Other ADSL services vary greatly, however they will always be limited to ADSL speeds. Present maximum is 2Mb/sec over ADSL, although the theoretical max is 8Mb/sec. Cable can support 10Mb/sec easily right now - obviously if everyone were connected at 10Mb/sec than the internal infrastructure would need to be improved.
My personal experience is that other ADSL companies can be ok, however many of them are fly by night and are tiny with utterly ridiculous tech support waiting time (over 1 hour on hold for tiscali (company changed hands three times while I was a customer) in the past to resolve an account issue).
Add to that the fact that when they have a technical fault, they have to then call BT then things start to go pear shaped.
I'd never recommend anyone choose BT for anything, ever. Rip off and crap.