I played a good bit more of Mario Tennis Open this evening and I have to say it is far better than I thought it would be, having read a fair few reviews - (games tm - 6/10, WTF??). I don't know where people have taken so much issue with this title. A lot of people have complained that the game has a dodgy single player structure, which is fair enough; the Game Boy versions had extensive RPG modes, but Open has some RPG elements and a whopping great online mode for itself! And it looks amazing
Whilst the previous handheld games had big single players, I don't think the lack of one of those in Open is that big a deal - many games aren't about big long campiagns otherwise no one would buy shmups, puzzlers, fighters or racing games like Mario Kart. For more "arcadey" games like your Mario Tennises of this world it all comes down to the gameplay - is the game fun, accessible and addictive? Is it fun over the long term to play? Is the multiplayer enjoyable? For me, on initial inspection at least, Mario Tennis Open would appear to be most, possibly all of the above, though of course I haven't played it in the long term yet. However, I have been playing the wonderous N64 iteration for a decade, and Open is as good as that in terms of gameplay by the looks of things.
If people go in expecting a big massive campaign then they will be disappointed - go in expecting a fun Mario sports game that is superb fun in both short and long burst and you may be pleasantly surprised.
Two things though - the online multiplayer is quite laggy right now. Nintendo need to patch this perhaps. Second, 3DS games are overpriced. I'll no doubt get my moneys' worth from this, but the asking price on this game is very high for regular gamers I think. Nintendo need to take this into account lest they get butt-hurt by Apple in the future.