I tried an earlier version and it crashed repeatedly and crashes happened very quickly. Although I swore I'd never try it again, based on some reports I read online, I tried the latest release candidate (1.18.4.2) and found it was actually stable and usable. I'm using a MacBook Pro 2Ghz Intel Core Duo (I) with 2Gigs Ram and OS X 10.4.10.

I've just installed the current release (1.18.4.3) and it seems to be stable on my machine. The list of bug fixes was very long (at around 50 items I stopped skimming), so they _seem_ to be moving fast on improvements.

The bottom lines seem to be:
1) Second Life stability is very dependent on your hardware combination.
2) If the current release causes problems, try the release candidate:
http://secondlife.com/community/downloads-optional.php

If you do decide to try it, take the time to go through all the tutorials. They're well worth the time.