We've had a recent extensive discussion about this on the flyer list - and the overall impression I got was that AOPA UK are about as much use to GA pilots as a chocolate teapot.
The discounts you get are pretty meaningless, and AOPA do very little to actively campaign on behalf of the GA community.
We're facing a crisis in the number of airfields under threat - and AOPA aren't doing as much to stop it as GAAC and Action for Airfields.
The JAA PPL is quite clearly a bizarre mishmash of poorly thought out legislation and poorly comunicated new requirements - and apparently AOPA just rolled over and let it happen.
I think that many of us concluded that if the sub was half the current price then we might consider joining and then trying to work within the organisation to get it to actually represent us better, but at the current price, AOPA is an irrelevance.
(Unlike AOPA US, which seems to be an admirable organisation that was in there pitching at a high level for GA within a few hours of the WTC horror)
For myself, I've not joined, but I'm going to join the PFA 'cos I reckon they've got my interests more at heart, and I support Action for Airfields as much as I can.
Steve R