Been using Quicksilver for years on both Leopard and now Snow Leopard... it's been pretty smooth. It might crash once every four months or something, at most - dunno why others are having so much trouble.
As for QS vs. Spotlight, yeah, Spotlight does the job, but:
On machines running Leopard/older PPC Macs, Spotlight is MUCH slower than QS
Spotlight requires me to first hit "Show All" before I can do anything with the results
Spotlight searches everything, which is good in some cases, but annoying when it turns up every little random application tucked away in the dark recesses of your HD
With a huge screen (27" iMac) I much prefer having my app launcher in the center of the screen
As for other alternatives:
- Butler: Don't like the way it's laid out. It's kinda like a weird Spotlight-QS hybrid. Also costs $20, which is a lot more than I'm willing to pay for this type of utility.
- Alfred: Much closer to QS and probably would be what I'd use if QS became unusable. Still prefer QS's icon-based look over Alfred's text-based one. Free (at least for now).
- LaunchBar: Haven't tried it, might be nice, but once again feels a little too much like Spotlight, based on screenshots. Another in the list that costs more than I will pay.
- Google QSB: Looks something like QS, but carries that same Google signature overinflated UI look. At least it's the right price.