Is Bristol as good as everyone says it is?
Frankly speaking "yes". If you don't use Bristol to help you prepare for the exams then unfortunately I suspect you'll be putting yourself at a huge disadvantage compared to everyone else.
Should you use Bristol QB as a substitution for actually KNOWING the subjects? Absolutely, positively not. As Paco points out, far too many people for far too long have used Bristol as their sole preparation for the exams. If by changing some of the questions, people are getting tripped up, then frankly - good.
There are some subjects where Bristol doesn't help so much (in terms of presenting the actual questions you get - such as Gen Nav), but even then, if you don't put the effort in to know the subject, you'll get destroyed in interview.