Personally I would recommend a Wii, as for me the games are really fun, and I like the control system (I never got the hang of the Ps2 style controllers - they always felt uncomfortable), but it's really worth trying it out first as I know some people who hate it.
I've currently got
Zelda Twilight Princess
Wii Play (if you can get it cheap it can give you the game + controller for only a pound or two more than the controller).
Metroid 3
Excite Truck
and Mario Galaxy
Of those Zelda is a great RPG/Adventure
Metroid looks very good so far, although it's not quite the same as the older metroids (slightly more shooting based).
Mario is rapidly getting me addicted - it's got just the right mix of classic platform action, with the addition of the 3d and controls for me (it takes a few minutes to get used to walking over the edge of the world, but it certainly adds to the fun).
Depending on the game the controls can work either wonderfully and intuitively, or very badly (IIRC EA basically ported a couple of Ps2 games over to it and spent next to no time adapting the controls for example*).
Metroid for example makes very good use of the controls, you use one to aim and interact with things, whilst the other controls movement and the grapple (the motion sensors take over the need for a second stick and a lot of buttons).
From what i've seen and heard it looks like there is definately going to be a shortage of Wii's in the UK this year.
*I think it was Tiger Woods golf that was considerably worse in it's controls to the Wii Sports golf mini game included with the console.