I spent a couple of hours with this last night. Impressions of the game compared to Donkey Konga--
--The song list is better than Donkey Konga's. It's a bit more... adult, I guess. The only semi-"kiddie" songs on it are the Dragon Ball Z and Jimmy Neutron themes, whereas it seems like about a third of the DK songs I'm embarrassed to play. Lady Marmalade, Material Girl, ABC, and Love Shack are fun, fun, fun, and as much as I hate Britney Spears, Toxic is one of the most fun tracks in the game.
--The covers are better than DK's. For the DK covers it seemed like the artists were trying and failing to do 100% accurate renditions--Taiko's covers either put a unique spin on the song, or nail it 100%. ABC, especially, sounds scarily like the Jackson Five.
--I think it's harder than DK. I was worried because the game doesn't distinguish between hitting the left side of the right side of the drum, and doesn't have clapping--the only "notes" are drum surface with one stick, drum edge with one stick, and the same options with two sticks (and it looks like you can cheat on the two-stick notes and only hit with one). But the difficulty comes when the game throws buckets of notes at you and you have to figure out the best way to hit them all with both hands. Sometimes hitting them all with one hand works; sometimes you have to alternate; sometimes neither of those will work. After having DK for a few weeks and only playing intermittently, I can easily clear about half the songs on the highest difficulty level, but with Taiko I have trouble clearing many songs on Normal, not to mention Hard. Oni I just look at and laugh.
--Part of this is because I'm very weak with my non-dominant hand, which is a feeling I haven't experienced in gaming in a long, long time (possibly the last time was when I first had to learn to use a console controller, with the analog stick for the left hand). My right hand is my writing hand, and I'm pretty good at doing fairly complex beats with it, but my left hand feels worthless half the time. I'm not going to be able to do well in the higher difficulty levels until I'm as comfortable drumming with my left hand as I am with my right. I have no such problems with Donkey Konga, oddly enough.
--The controller: I got the game at EB, and for $3 they'll give you a year's warranty on the controller, so that you can bring it back and get a new one, no questions asked. This is a good thing, because you'll tend to smack the hell out of it--the gentle tapping that you can use for the DK controller won't cut it here. Unlike the DK controller, the sweet spot for hitting notes doesn't appear to cover the whole surface of the drumhead--there's a fairly large dead spot in the middle, and I'm not quite sure where the sweet spots on the edges are. It feels fairly robust and very precise, though, and when I miss notes I feel like it's my fault, not the controller's (though a fair number of my missed notes come from me not hitting the sweet spots on the left edge, due to my aforementioned gimp left hand). In comparison, the DK drums have no dead spots, but there's the clapping to deal with, and the subsequent stinging hands after a marathon.
--The graphics are much better than DK's. DK's visuals always seemed like a hack job to me--Taiko's graphics are much more readable and polished from top to bottom. And there's the aforementioned off-the-charts cuteness factor. Now is as good a time as any to mention the localization--between this and Katamari Damacy, whoever is handling localization at Namco deserves a serious Christmas bonus. Just enough tinkering so that the game makes sense in English, and then leave it the hell alone--perfect.
--The minigames appear to be just as worthless as DK's: no news there.
--I haven't tried the multiplayer, but I'm guessing that DK will be the better multiplayer game, just because it's easier and more accessible. DK's superior multiplayer component almost completely mitigates any flaws it has when compared to Taiko. It doesn't look like you can buy standalone Taiko controllers either.
I'd say that if you have both a PS2 and a GC, you can't go wrong with either Taiko or DK: they both have their advantages. If you know you're going to be playing mostly alone, though, I'd lean toward getting Taiko, which has a superior single-player game.
The songs are much longer than Donkey Konga as well.