Just before my recent travels Mail began crashing on me in 10.5.7. When I launched the application it would quit immediately. While webmail is tolerable, it becomes annoying when you have more than one account to check. There's simply no substitute for a good, reliable Mail client.
It's amazing how dependent I've become on Mail.app. I might not notice if the mailman didn't show up for a week, but I start climbing the walls if I'm without solid email access for more than 12 hours.
Things to check when Mail unexpectedly quits on launch:
I didn't have any Mail plug-ins installed and resetting and repairing didn't help either. The only thing even remotely resembling my situation that I could find is this thread started yesterday on the Apple Discussions board called Mail Unexpectedly Quitting Constantly in 10.5.7.
Here's what fixed it for me: An archive and install of Mac OS X with the silk-screened DVD that came with my MacBook Pro. Now Mail is running again, but I'm back to 10.5.5 because that's what came with my MBP. Unfortunately most of Apple's latest apps (i.e. iTunes 8.2) require that you have 10.5.7 installed.
Update: Mail works fine after installing the 10.5.7 Combo update as expected. Software Updates shows that I have a litany of updates pending, but I'm only going to install QuickTime and iTunes 8.2 in anticipation of iPhone 3.0 requiring them. I'm going to hold off on installing everything that's not 100 percent essential for the time being.