I just recently purchased a palmOne Zire 72 to replace my old HandEra 330 which died a while ago. I think PDAs are great devices. On a simple level, they can be your calendar/date book, address book, to do list, and memo pad all in one. That alone may be worth buying a basic device. Add applications and you can do plenty more. For example, I use Handy Shopper (free) to make my grocery list and use the PDA every week when I go shopping. I recently installed Progect (free), a sort of outliner/to-do database project manager, to help me keep track of tasks I need to do at my new job and how far I've gotten along with them. And of course I've got some games and e-books for entertainment, as well as a 1GB card with MP3s ripped from a bunch of CDs I own. My PDA goes pretty much everywhere I do because I can use it for so many things.
Are you asking about the Zire 31 specifically or PDAs in general? That particular model is a lower end one, you can compare it to other models for more details and info, but if you don't plan to do anything too heavy on it, it's probably fine. Since you mention MP3s, you should note that an expansion card is required for that model (same for my Zire 72) -- plan to spend a bit of money for at least one; a 1GB card should run in the area of $100 or so. On a general PDA note, I think you should at least be aware that there are also Pocket PC devices that basically run a version of Windows, IIRC, in addition to the PalmOS devices like the Zire models. Personally, I didn't want a little mini-computer, so I opted for a PalmOS device. Depending on what you want to do, a Pocket PC device may meet your needs better. If you can give us some more info about what you're planning to do with a PDA, maybe we can offer some more/better suggestions.