I've used three of the four (the Contintental being the one I haven't managed).

US Airways - big, sterile (but clean and tidy) and the bar is upstairs (I was in at 6.20am, so not really "thirsty" in that sense), poss because of the over-21 rule.

The "upstairs" Servisair one (can't remember which one - sorry!), same level as the US and Continental lounges. A hell-hole. Small, cramped and usually over-ran with screaming, shell-suited children.

The "downstairs" Servisair one, same level as the shops - no under 12s, and a more spacious, cleaner environment make it the clear winner in the Servisair stakes.

If you're restricted to the last two (e.g. if booking via Easyjet/BAA/Holiday Extras etc), then study them carefully. They're not as good as an actual airline lounge, but anything is better than the wilds of the South Terminal!