I'm running a MacPro 2.66 and so far have had few if any problems. I suppose most people who are having trouble it's probably machine related. I notice that most of the complaining is coming from iMacs or people who don't know how to use the software.
The only major issue I've had is Mail. In Tiger, I finally had it configured to catch most junk mail. In Leopard, I lost those settings and have had to go back and reconfigure it. Also, I haven't been able to empty the junk mail folder without selecting all of the junk mail headers and hitting the delete key. In Tiger, all I had to do was "right" click on the junk mail folder and choose delete junk mail. Now the option to delete junk mail is grayed out even if the folder is selected and contains junk mail.
Buggiest software ever? No. A few issues here and there... yes.