I'm a Nintendo loyalist, but I agree with Spuds, they're notorious for making you buy extra crap to play a game properly. Many GCN titles required a GBA, which I never had - I couldn't justify the expense.
Now at least I've got a DS, so when those kind of games come out for Wii, I'll be prepared. (And I feel as though the DS was worth the scratch, while the GBA wasn't really, for me, although I'm told it was a top notch doohicky, and I'm playing a few of its games on my DS.)
I think Spuds, that overall, since GCN, Nintendo has been producing successes pretty consistantly. They're on a roll, and I think it will keep up for at least the life of the Wii. By the way, I don't expect too many extra peripherals for the Wii, because the nature of its controller fills a lot of the needs that might arise. You could play a game like Donkey Konga with two Wii-motes, no actual drums needed, etc.
We'll see though. I usually avoid games that require extra peripherals (although Guitar Hero is so cool, I just might buy it). I don't really see the point of the gun attachment, as the Wii-mote can already behave as a gun. It's just a cosmetic difference looks like, and I think you'll be able to play games with or without it. As for the classic controller attachment. I think that it will be good and not too pricey (and definitely worth the price, unless you can use GCN controllers instead, then I doubt I'll bother).