I was expecting a bit of over touristy stuff around Torres del Paine, but its not too hard to avoid and I thoroughly recommend the walk. I did the 8 day full circuit with valley side trips in early April 2005 and saw fewer than 5 people a day on the "backside" of the circuit. The front "w" was a little more crowded but not painfully so.
Tips:
-go as late as possible. The tourist numbers drop and the deciduous beech are turning orange. April was great. Also hardly had any severe wind, and had rain only for a few hours one day.
-Do the full circuit. Its worth it. Scenery changes dramatically day to day.
- Leave time for a side trip up the Valle Frances. Worth a day, or camp overnight.
- I camped. there are cabins most of the way
- good infrastructure in Puerto Natales for arranging the walk and buying permits etc
The area around El Chalten in Argentina also has great walking. The side trip up to Lago Electrico is well worth it. But the Torres circuit is probably more classic- given that its longer, and a circuit. I hear there is very good walking around Ushaia as well.