I have a 501W that I got 10 months ago and I really love the light, unfortunately I had to send it back under warranty. It would fail to start reliably. I think the problem is related to the end cap, which you twist to stop the light from going on inadvertently in your pocket. When I would screw the cap back in, the light would not start unless I removed the battery, waited several minutes and then reinserted the battery. Perhaps the connection in the end cap is not quite what it should be or the electronics has some issue. Very weird though and not related to the switch AFAIK.
Anyway, like I said, I love the form factor and utility of this light and am looking forward to getting a new one under warranty. I also like the form of the light so much I probably will get one of the H51 headlamps as well in the near future. I just hope I don't have the same problems.
EDIT:
Received my repaired H501W in the mail today. Looks like they may have replaced the tail cap. Its working now, although I seem to have to screw the tail cap tighter to get it to run. I really love this light though. As others have said, there isn't another manufacturer who makes as nice a headlamp/angle pocket light that I know of. I ended up ordering an H51 today.