There are a lot of WebOS fans around here. Even Apple has hired back a few of its designers to help with the iPhone.
Its pros are a wonderfully intuitive multitasking UI, a terrific notification system, nice integration of mail, social items, etc... and it's just fun to use.
Its cons right now are: smaller screen than most newer phones, slower hardware than newer phones, and far fewer apps.
If Palm uses HP money to create a state-of-the-art hardware platform, WebOS will get another chance to shine.
My main advice if you do try the Pre is this: realize that it might take a few days to get into the swing of a new OS/UI paradigm. Don't give up. Do read WebOS forums for beginner tips. (This is true for all phones.)
Personally, I like the Pixi better, even with its smaller screen. No need to slide the keyboard up/down, and it's incredibly slim.
Good luck, whatever!