Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Look, I think your buying decision with regards to Mario Golf: Toadstool Tour is best handled by a series of questions:
Do you want an arcade-style golf game that can be easy enough for your 5-year old kid (of nephew, or niece, or neighbour's kid that always comes to your place because you're the cool guy with the video game console) to play, yet allow a little more advanced (and potentially precise) style for you?
Do you like competitive party games that is played at a more leisurely pace than many of the others out there?
Do you like golf courses and challenges that are a bit outside the normal, with bright colours and plenty of stuff from the Mario universe?
Do you like funny cartoon-style animations whenever someone puts the ball either under or over par, and with 15 initial characters to choose from (plus four unlockables) to have quite a range of so (and which of course can be cut short when you tire of them)?
Basically, if the answers to three of these four questions are yes, then this is a very good game, especially now that you can find it in the bargain bin. There won't be any real surprises here, but the challenges are plentyful, and with enough variety to keep it going. And everything (graphics, music, sound effects) does the job, but not much more. The aforementioned characters are in fact different enough in playstyle to make a difference, especially when you unlock their "star" versions. Though it's mainly the maximum length they can hit the ball with that changes.
To mention the negative stuff: As a single-player game for a 10-year old or older, it definitely leaves something to be desired, because it's too simplistic... But then again, golf isn't really the kind of sport that leaves itself well to a single-playergame to begin with. And the camera control can be rather... well, uncontrollable, really.
But again, it's the kind of game you should, with the said simple questions, know whether or not you'll enjoy before you buy it. And as an uncle to a 5-year old kid, I certainly do, though much of the enjoyment is watching him go utterly bananas when he makes a birdie. Not to mention the time he got a hole-in-one...