I completed my ATPL exams via Bristol last month. I came out of them with an average of 95% after studying for just over a year while working full time.
I cannot directly compare the Bristol course with any other as I have only taken the BGS course but I will say that the whole experience has been a very positive one. The materials are excellent, the support is very good when you are studying and the two week brush up courses before the exams are a revelation. The instructors are a good mix of guys from a wide cross section of the aviation industry as all seem to have a real vested interest in getting you through the exams.
If you want more information or to talk through anything else PM me!