I'm not a big fan as past experiences have been too lopsided. Far more English spoken than Japanese so it put me off for quite a while. However a few weeks ago, when I went back home to Australia I met up with a Japanese friend, and most of the conversation was in Japanese. It wasn't really organised as a language exchange but I kind of felt a bit guilty afterwards. Also, since I've just moved to a new city (Sapporo), I'm actually just trying to organise a few language exchanges now (got one tonight I think). My Japanese is much better these days than a year or two ago when I first started doing language exchanges, so it might turn out to be a bit better. We'll see.
Also, from past experience, if your Japanese language ability is too good, then many Japanese will not want to meet you. They'd rather meet someone who is a complete beginner, as this guarantees them ample English time. So now when I contact someone, or am contacted, I write to them in the best, most formal Japanese I can, just so the leeches are weeded out from the very beginning and I'm not wasting my own time.