I have a sim in my basement -- convinced my wife we needed to do it when we finished it last year...which included engineering and structural work to raise the ceiling.
I've had both a Skytrak and a Foresight GC2+HMT (what I currently use). Love the Foresight -- very accurate and never misses a shot. With GC2+HMT you get the same info as a GCQuad like the pros use, but it lacks the portability (which is fine for me). The Skytrak is a solid unit, but you do get what you pay for.
The big thing I recommend to folks thinking about a sim is to put in cameras. This has been by far the most helpful thing to my game. Being able to see your swing and get instant feedback is very powerful.
Matt