I did the CBIR with Booker Aviation last month and rate them highly - very accommodating.
I am, however, finding the process of dealing with the CAA to be horrific and, in many ways, harder than the test - that's the subject of a whole other post. Had I known how difficult and expensive that aspect of it would have been, and how they seem to go out of their way to be unhelpful, I might not have bothered with the rating in the first place. Red tape challenge my @£$&.
There are various different routes to the CBIR and you should make sure that your ATO is fully aware of the requirements for your specific route and that they help you to prepare accordingly.