Previous ISP 1 - ntl (cable)
Speed - 2mbps (what would now be the 4mbps service)
Cost - £24.99/month
Usage Cap - Used to be 30GB/month IIRC, then became "unlimited" (subject to FUP).
Quality of Service - very stable & reliable connection for 5 years (bar a couple of problems), the customer service was often very poor.
Previous ISP 2 - Firefly (ADSL)
Speed - 2mbps (I didn't regrade to "up to" 8mbps RADSL/Max, as by the time that had arrived, I decided to wait for the contract to run out & switch to Sky)
Cost - £24.99/month
Usage cap - 30GB/month (& £1 per extra GB over the 30 IIRC)
Quality of Service - quite stable & reliable connection, excellent customer service.
Current ISP - Sky Broadband Max (ADSL2+)
Speed - "up to" 16mbps, currently ~ 8mbps (line distance/quality)
Cost - £10/month

Usage cap - "unlimited" (subject to FUP).
Quality of Service - some issues so far with stability, but seems to be improving. Not officially used CS/TS yet so can't rate them.