I personally think it's worth it, but I have to echo what Chris says about making sure to weigh up your own needs, the cost, and all the rest. After all, like anything, it's not something that's going to be suited to absolutely everybody, and for some folks there are going to be better options out there for them.
I'm afraid I can't compare to the Wiz, though - unfortunately I was burned by the early problems that the GP2X had, and didn't want to purchase another product from the same company.
Anyway, just in case it's still of any interest, myself, I use mine for gaming primarily, but I also have a 3G/HSDPA "MiFi" router (a pocket-sized router that some cellular phone providers in some countries offer) which I use for internet access when travelling with it. It also serves as an audio player when I'm on the move.
If you're curious, my most-used apps (in no particular order) are as follows;
Snes9x4D4P (The SNES is my favourite system of all time, and this is my favourite SNES emulator)
GINGE (GP2X compatibility application - it allows many GP2X games and applications to be run)
GPFCE (A GP2X NES emulator, used via GINGE - it's currently the best option for NES emulation
)
Cave Story (The GP2X version of this wonderful game can be run via GINGE)
C64 VICE (A Commodore 64 emulator)
MAME4All (A speedy, well-optimised version of the Multiple Arcade Machine Emulator, which also features high score saving)
MAME 0.106 (A build of a more recent version of MAME, for the games I own that MAME4All doesn't support)
Pandora-2600 (A port of the Atari 2600 emulator, Stella)
Pandora-Atari (Emulator for the Atari 8-bit computer family)
O2EM (Magnavox Odyssey 2/Philips Videopac emulator)
DOSBox (Well-known application for running MS-DOS software - works very well for the games I own)
PicoDrive (Mega Drive emulator)
Deadbeef (Audio player)
Mousepad (Built-in text editor - I use my Pandora for writing on occasion)
Firefox (PND version of the desktop web browser)
There are others, of course, but those should probably give some insight into what I think mine's good at.
Some things occasionally need overclocking (which is NOT guaranteed - it should just be considered a bonus if yours can), but the need for this will probably disappear as things are optimised.