I also had the same concerns when I was switching from my 755p. I was very disappointed with the PIM management and how apparently slow the phone was (before homebrew). I was a very reluctant convert and waited quite a while before getting the Pre Plus. What you have to remember is that you aren't going to use webOS the same way you use Garnet. While Garnet has some really fantastic highpoints (super fast, excellent PIM management), it also does not multitask--and multitasking is the key to using webOS.
It took me over a year to get it through my head that it is OK to leave cards open all day long, and that that's what the cards are for. Now I keep my calendar and other most used apps open all day, and that makes appointment entries, etc, super fast. Plus I can do all that while loading a YouTube video.
For tasks I use ToDo Classic. It's the closest app I've found to the Palm tasks. The others are a bit too clunky and cluttered for me.
I use the stock Tasks app as my shopping list. It's fantastic. I use a new category for each type of store (grocery, hadware, craft, etc) so when I'm at that store I know at a glance what I need. What is really cool is I can set a due date on an item. For example, I have a due date set on birthday cards a week before the person's birthday. When it's time for me to buy and mail it, it pops up in the notification area. It's very cool!
But besides all that, you also have to consider the connectivity. Internet 24/7, including wi-fi. (No Verizon Garnet device was afforded that luxury.)
So I said all that to say that the benefits of webOS, I think, outweigh the cons. You just have to rethink your mobile habits. I would also suggest you listen to PreNotes to really get to know what the phone can do and how to do the little things that make the phone truly smoke:
preNotes | | webOSroundupwebOSroundup
Hope that helps!