Ha ha ha... welcome to my world. I've owned a Palm Vx, Toshiba e800, Palm T3, HP iPaq 4150... and now, a BlackBerry 7250. The sad part is, I've still have almost all of them. Some were compatible with various computers I went through, others were not.
First: Check to make sure whatever you want can sync with a Mac. Scope out MissingSync and a few others.
Second: Ask yourself if you want just an organizer or if you want to do other things (take notes, etc.).
Third: Realize that if you have a good laptop all the time or always have pen and paper handy, you will almost NEVER use a PDA to take notes. Why? They're just too small. You can type faster or write on paper more comfortably.
Fourth: If you do decide to get something, go SMALL as you can hold comfortably. The larger the device, the dumber it looks on your belt or the less likely you're going to take it with you.
So, if you were to ask me what I REALLY need, I'd have to say that my old Palm Vx from 5 years ago still would be enough. It has my calender, addresses, etc. and I could take the OCCASIONAL note if I really wanted to.
But if you were to ask me what I've found the most USEFUL, I'd have to say the BlackBerry. It can't hold large files (no memory expansion) and it can't play music (damn!), but it does email like no other thing around. And it does have the usual calender, addresses, notes, etc. So, it's sort of like my old Palm, but with the added benefit of email, web, and phone. It's got some little games, but I hate video games, so I don't care much for that.
For me, these are my priorities for portable gadgets:
1. Communications (cell phone for emergencies, but more importantly, EMAIL on the go - more important if you travel).
2. PIM (address, calender, tasks).
3. Entertainment (for me, music and audio books)
If you're a Geek and gadget nut, by all means go for the latest PDA. Realize you cannot fulfill (1) above unless you get a smartphone (Treo, etc.).
If you're practical, go for a cellphone or BlackBerry. It'll fulfill (1), (2), and maybe (3), depending on what you get. Or, just get an iPod: it'll fulfill (2) and (3), but not (1). Just realize that you can't change things on the go with an iPod, but this is only going to likely happen less than 20% of the time.
If you want it all and are willing to give up a few things, get an iPod AND a cellphone: you'll sacrifice some portability as you now need two devices. (A Nano, on the other hand, would be nice and small.) Or, get an all-in-one smartphone (e.g., Treo, a few Windows SmartPhones), but realize they can be quirky, or are missing some features, or are just plain expensive.
Check out
http://www.brighthand.com for a great forum on PDA type devices.