I live and swear by my PDA! I started using a Palm Vx about 8 years ago and was instantly hooked. Upgraded to a Treo 600, and I've just about worn out my Treo 700p. It is just sooo incredibly handy. The search button alone is worth the price- how many hours wasted looking through old notebooks and day planners for that one scribble? Also, it's excellent for tracking contacts- I put key words for people in the notes section so that when I forget their name, but remember their employer, or a project I worked with them on, etc, I can search for that and pick them out of a short list.
I'd highly recommend getting a PDA phone as opposed to a standalone; a PDA isn't something you use "some of the time;" a PDA is useless unless it it *always* with you. And having it converged with your phone is doubly convenient. Since they're all also cameras/camcorders, you'll have that with you all the time, too. And wireless email/internet just kicks ass
PDAs pretty much all support Word and Excel these days, too. A lot will view powerpoint and pdf, too. I even have a CAD package for my old Treo, though it wasn't very practical. I'd also recommend one with an SD slot; you can pick up a cheap SD card reader and use it like a thumb drive to transfer files to/from your PDA and laptops.
As for cases- anything you can imagine, there's a case for it. I had a solid aluminum hardshell for my old Treo (had a nice leather case, too), but I broke too many belt clips, so I just slip it in my pocket now, "naked". I have a little friction sticky on the back (free swag from an Eaton junk mailer, actually) so I can just set it down in my car and it doesn't slip while I'm driving.
A PDA isn't for everyone, but if you use a day planner and you're a gadget guy... it's WELL worth it! Also look at Pocket PCs and Blackberrys. IMHO, PalmOS is by far the best at being a PDA, but PocketPCs are better at being a toy. Blackberrys are cheap for the back-end support so businesses love them, but I'd never buy one for myslf, get a Palm or PocketPC instead. In the end, though, they all do pretty much the same thing, it's just what you prefer. There's tons of 3rd party software for all of them. The IR beaming is mostly compatible between them, too, for sharing files. (Anything in a Palm can be beamed to another Palm)
