An AQR day made the single biggest difference to my riding in about 10 years, despite the fact I'd been riding with one of the guys for most of that time. Mostly a question of sorting out my bad habits, but my cornering came on a mahoosive amount as a result.
I've also done a Nigel Page day. That was a) an awful lot more expensive, b) an awful lot more esoteric. I came away from the day going "?????" quite a lot, but much of the stuff he taught me fell into place over the rest of the summer.
Overall the AQR stuff probably made me a 20% better rider, the NP day added maybe another 5% capability onto the top of that. (but it's that 5% that's got me through some damn scary stuff)
(please note, I am a bit biased as AQR are a bunch of my mates)