Have a couple of observations to make:
Regarding the post from BassBoy i'd pretty much agree with most of it but would like to clarify a couple of points.
The course assumes that you have a good deal of experience and knowledge and as such, the emphasis should be more on testing you than teaching you.
I'd agree with this statement insofar as the statement made in the RYA Powerboat Logbook is
Candidates should be competent to the standard of the intermediate powerboat certificate with a thorough knowledge of navigation and chartwork to the level of Day Skipper Shorebased Certificate
However i would be keen to emphasise that this does not mean that the
emphasis should be more on testing you than teaching you.[/
Yes of course there is a need to ensure that you are up to standard on the navigation and chartwork side and of course your boat handling should be very good already but the Advanced Course is very much just that - a course - and as such that's the way an Instructor should approach it with you. After all there are plenty of subjects introduced at this level that will not have been covered before and may be new to you.
Finally i would very much agree that it is key to ensure that you are happy with the people/school that you are going to deal with - speak to them, visit them and put them through their paces. If they survive that they deserve your investment!!
Regarding PeterBs question re STCW95 medicals in my experience the STCW95 ones are not time limited. That said everyone i know who has one works on the basis of a 3 or 5 year update to keep abrest of changes.