I have a Handspring Visor Pro, and it has been quite a work horse for me. Although, I've seen several new models from other vendors that look quite appealing. If I was looking to buy a new PDA, here are a few funtional things I'd look for now - roughly in order of importance:
Size
- What size will work best to help you use it the most?
My Visor Pro is a bit bulky (mostly because of the Springboard slot), and it works fine in my Franklin Planner and a fanny pack. It doesn't work so well in a pocket.
If the PDA is going to be carried in a purse, backpack, or other bag, a slightly thicker model (usually less expensive) may work just fine. Whereas, thinner is better if the plan is to carry it in a pocket.
Memory
- What will the PDA be used for?
If just for addys, phone #'s, and appts, 2 - 8 MB would be plenty, depending on other little software add-ons.
If for data bases, files, email, ebooks, apps, etc., 16 MB is best.
Expansion
- Can you add more memory/functionality?
The most common addition to a PDA that I have seen is memory - whether SmartMedia (SM), CompactFlash (CF), Memory Stick (MS), Secure Digital (SD), or MultiMediaCard (MMC).
The Handspring Springboard slot allows for adapters for all of these technologies, so you can use the same memory as the other digital devices in your household, if you have them. Sony uses their proprietary MS technology. Palm, the Handspring Treo 90, and other vendors typically use the SD/MMC technology, which seems to be the direction many digital devices are moving.
Personally, I have a 128 MB Compact Flash card and an MemPlug on my Visor, and I keep a fairly large library of ebooks and vector street level maps of CA, NV, AZ, OR, and WA for my GPS. Given my druthers, I'd probably go with a 256 MB SD card now, but that wasn't an option at the time.
As for other expansion, like GPS, I have a Garmin 12CX GPS that I connect to my Visor via serial cable, and I have QuoVadis running on my Visor. This set up works quite well, and it allows me to put the GPS in the window of the car (for best reception) and keep the Visor near by for trip monitoring (speed, heading, etc.), mapping, and GPS operational status.
I haven't seen many raves about the various Springboard GPS systems, so I'd recommend getting an inexpensive stand-alone GPS with WAAS and cable it to the PDA for the most flexibility. Don't underestimate the power of the USB/Serial port on most PDAs - the Springboard isn't the only way to go.
With regards to cell phones, I think Andrew made the point clear. The Springboard just doesn't quite work. I'd rather have my spiffy little Nokia 3395 and relatively big Visor than either a PDA with additional hardware attached to it or a cell phone with a tiny Palm OS screen. I don't really need wireless internet, and I don't want my e-mail while I'm out - if people want me, they can call me... 
Screen Resolution
- What do you need to see on a daily basis?
For basic stuff, addys, phone #'s, etc. 160 x 160 is quite tolerable.
For GPS mapping, photos, drawings, etc., 320 x 320 is almost a must. I've been getting by on my 160 x 160, but I'd sure like that extra resolution to make things more crisp and clear. More and more apps are starting to take advantage of this increased resolution.
Color
- Do the apps you plan to use actually take advantage of color?
Addys, phone numbers, ebooks, and most appointment apps don't need or take advantage of color. Sure, the icons on the launcher look better, but is it really worth the extra money? Graphic apps, like GPS mapping, astronomical apps, digital pictures, and games, really utilize color to its full advantage.
Also think about how the color is produced. Some color screens do not work well in bright light or outdoors. B&W screens typically work well under all conditions, although backlighting in the dark usually requires it to be very dark.
Right now, all of the work and play I do with my Visor works just fine in B&W, but I do have a couple of apps that I'd love to see in color.
Data Entry
- What is the most comfortable way for you to enter data?
Generally, Palm OS PDAs utilize the Graffiti handwriting recognition technology, which requires you to "relearn" how to write. Some people find this a pain, others adapt easily. The new Handspring Treo line has changed that paradigm and gone retro, and they now offer a QWERTY chicklet keyboard on the PDA for you to type with. I've never used it, so I don't know how easy it is to use.
I'm fine with Graffiti, and if I need to do a lot of data entry, I do it on my Mac or PC and sync it. Another option which several people find helpful is a full size keyboard which attaches to the PDA. I know Lani uses her's, but I've never been moved to get one.
Finally wrapping it up...
So, there are a quite a few options, but I think most people can cut through them pretty quickly. It is probably easier for a new user to decide than an e-geek who is looking to upgrade some time in the future... 
Good luck, and most importantly, be sure the one you decide on is a good fit and fun to use. If it is a pain in the rear to use, it'll just collect dust.
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