Last one I went to, the other night, was near Swindon and was pretty wretched. Typified by a guy suggesting a song that nobody knew, and then playing it anyway.
Best one I've been to was the Blues jam at the Lagerhalle in Osnabruk. It's been going for over 30 years and is well attended and also features some of Germany's finest players.
Players get fed and watered too.