I haven't played it very much, but it is a decent game. Not great, but with the coupon it's only $21.88 and probably worth it if you like the series. I'm not a huge fan of what they've done to the graphics (made it more cartoon-y, if that's possible) but it looks very good on the DS. The mini-games are great, too (air hockey is definitely a winner although seems pretty hard although I only gave it 5-10 minutes of time). [I know when I got SMB for the GBA, I spent more time on Monkey Bowling than anything!!]
The stylus control is kinda 'eh'. I find on some levels, the stylus works better, and on some the d-pad works better. I guess that adds to the challenge. Some of the levels are incredibly hard-- you need to use the stylus for 'analog' but it's still not quite precise enough for some of the things you need to do.
It's a decent game. If you like the series (as I do), definitely worth $21.88. If you're a casual fan, or never picked it up, it may be a bit more questionable (not the best introduction to the game).
I'm glad I got up early to go to Target yesterday morning to get it.
(my local Target is a ZOO if you go too late on weekends) Even saw 2 Xbox 360 core units sitting there. Weird!