I've run the past 3 years with the local BMW club at Mosport - quite a bit of fun and very instructive. The BMW club runs these events as a "performance driving school", not a racing school, so your insurance is valid and you get a tax deduction (at least in Canada) because it counts as an educational expense! The flip side is that you must attend the classroom and skidpad sessions - I way 'flip side' because I've found the quality of the classroom sessions to be a bit spotty. Same complaint from several people I've met who have run at MidOhio and the Glen with the BMW and Porsche clubs. The skid pad sessions are useful for pushing your car to the limit in a benign environment, but there isn't typically enough opportunity to do the same 'course' often enough to extract the maximum learning. At any rate, there are cheaper ways to get more open track time (in particular open lapping sessions), but I'm not aware of any that allow you to rely on your insurance, that provide one on one in-car instruction, and a fully marshalled course - personally I wouldn't give up any of these for any amount of cost savings.
Eric