I always go with the Tacx range, good turbos and value for money.
Depends what your budget is though as turbos can be bought from around £70 - £1k.
I've got an old fan assissted one (no longer made by Tacx)...the faster you pedal, the more resistance. Therefore, you've got to use your gears to get a decent workout. I've also bought the Tacx Satori although not used in anger yet. Beauty of this is it folds up so I can take it to events for warming up, etc.
I'm thinking of getting the Forteus Multiplayer but still undecided on that yet.
Have a look at the Tacx range:
http://www.tacx.com/producten.php?language=EN&fl=true&lvlMain=16