I bought the OAT manuals about 8 months ago to study whilst saving money, building some hours and losing a bit of weight. I've just enrolled with Bristol, got their manuals this week.
Doing the background reading was useful, if you have time to kill like I did then why not use it to study? That said I could probably of learned as much from three or four commerical text books and saved a few quid. I've seen various books of this type for sale with Transair and Pilot Warehouse.
The OAT books do have a lot of questions, and you can never have too many of those. They'll be useful for cross checking the Bristol notes.
The Bristol notes do seem very good - they are simpler to understand than the OAT notes.
Good luck.
Karl.