I've been using the Palm TX for GTD for about 2 years now and like yourself, that's the only thing I use it for.
Lists: NAs, Projects, Someday/Maybe, Waiting for, etc
These are easy because there's a program called Docs To Go that allows easy syncing with your PC at work and/or home. So you can edit the lists when you're sitting at your computer or on the go.
Calendar/Ticker file:
I use the calendar on the Palm as my GTD calendar and tickler file. The calendar syncs with Yahoo calendar, so when I'm in front a computer I review and update the calendar on Yahoo, and when I'm on the go I use the Palm.
I wouldn't worry about screen protectors etc - this thing is meant to make your life easier not add more hassles. I've been throwing it in my jeans pocket for years now and it's very robust, haven't had any problems.
I'd definitely recommend getting this Palm if you don't mind paying for lots of features then only using 10% of them (like me)!