I currently use BTInternet Surf package and connect on average at 48k bps. Not noticed any particular problems or issues over the last few months, but the 2hr cut off is a tad annoying at best.
Did try freeserve before that, but connection speeds ranging from 35k bps to 42k bps (never higher than 42k bps!).
If you can I would recommend going broadband which is available through a load of cable operators for something like £25 a month all in. Most of my friends who have this service cannot fault it (when it eventually gets installed!).