I've been with TTB, including its previous manifestations as Opal and Nildram before that, for at least 8 years.
Like most of the responders my experience has been positive, prices and service are good. At weekends the helpdesk very occasionally responds with a foreign accent, but almost invariably they are obviously UK based and speakers; always courteous, knowledgeable and helpful. With regard to speed, I'm on ADSL2+, and currently get a shade under 9MB download, more than adequate for my needs. Fibre is available, but I'm holding off for the moment.
The only problem I've encountered is that their help pages (FAQs) are a useless joke. As an example, I recently wanted to forward our incoming phone calls, for which instructions should be easily locatable on their site (imho). I ended up ringing the helpline in disbelief and considerable frustation but spoke to an extremely helpful lady who talked me through the (very simple) process and emailed me a copy of the instructions.
It was a pleasure to deal with this operator, but it was nonetheless a call that should not have been necessary.
That said, I would suggest go for it - I don't think you would be disappointed.