Rightio, I just beat Mario Tennis Open in singles so I'm going review it and then shut up about it
I think the best way to review it is to break it up into its consituent parts; presentation, mechanics and strucure, and gameplay. Please note I have only had the game a few days, but I am a series veteran. On with the review....
Presentation:In my opinion, this is absolutely beautiful to look at. Nintendo sure knows how to create and oversee fantastic looking video games. The visuals on display here are top notch - rich, bright colours, rock solid frame rate and slick animation. The characters are full of life and often very funny looking. The way Diddy serves the ball with his tail is brilliant. I also love all the little touches like the fire "coming out" of the screen on the Bowser court, the way the ball genuinely leaves the leaves suspended stages and flies off into the ether. And all the little touches - I love how Toad, the umpire, is suspended in the air above the court in a cage on the aforementioned Bowser level - great stuff!
The music is your regular orcastral fare from Nintendo. Nothing quite as good as "The Dog's Theme" from Kid Icarus Uprising for example, but there are some nice tunes. I really like the character voices too - the cast never shut up taunting each other. Peach gets a bit annoying though - but Waluigi is fantastic.
The menus in the game are all easy to navigate and the e-manual is an improvement on others Nintendo have done lately. I would MUCH rather a physical book but the pamphlet and e-manual are reasonably good.
Overall, this is a very slick package. The video at the start of the game sets the scene nicely for what is a sumptuous looking game of tennis. About as good as a game like this can be. But some don't like Mario games' look, so this obviously won't do anything to change those peoples' minds... can't please everyone I guess.
Mechanics and structure:In terms of control, this is similar to the others in the series in that all the previous standard moves are present and correct. However, in terms of controls, the 3DS's triggers aren't exactly ideal for shot cancels and the dive move. This isn't a huge issue, but the N64 control set-up, which was 100% perfect, isn't quite as good here. Still very good all round though and the analogue stick is superb.
In terms of balance the characters aren't quite right I suspect. For example, I had huge problems trying to beat Wario with Yoshi in one of the latter cups. It was really really hard and I gave up. I tried Donkey Kong and in the same stage and I beat the competition - this time Boo - easily. Maybe Yoshi's super fast speed just didn't suit my gaming style, but Wario was very strong and quite quick too, whilst Yoshi's shots were weak. Perhaps some gamers might be able to utilise Yoshi better than me? Only with a lot of play will I be able to comment further on the balance of characters, but for the time being I would put a question mark over the balance of the participants.
Quite a few people have complained about that the game's structure is crap. I agree that the lack of a long RPG mode is a weak point to the game, but as I mentioned before, the additional online mode and the fact that there is quite a big Mii customisation option is enough to make up for this, for me at least. There are lots of modes and options in the game, such as the "Super Mario Tennis" game, five adjustable difficulty levels, lots of tournaments for single and doubles, secret characters, and a magnificent local multiplayer option. I personally think there is lots of content here in what is a tennis game after all - what more could be included really? Plus the actual difficulty of the tournaments is very high at the end - you will need to be very good to finish the game - it took me about 8 hours only to beat it, but that included repeat matches and practice. You won't beat the whole game in a single sitting, unlike what some would have you believe. Still, 8 hours is fairly average for games these days on your first play through, and unlike many other games out there days, Mario Tennis Open has a lot of replayability.
I have had issues of lag in the online game. Plus it doesn't have a lot of options - but this is early days for Nintendo's online offering. Fingers crossed that the lag is fixed at some point soon. With time I will have more things to say about this I feel.
Gameplay:Ah, the gaemplay

This is where Mario Tennis really shines. First, the changes over previous games - out go the stupid power shots of the Power games, and in come the Chance shots. I though these would be rubbish, but there are actually really good - surprisingly so. They power up regular shots basically when you hit the sgot on a chance pad, which appear every now and then on the court. The shot is returnable by the other character if they know what they are doing, so they are not overpowered. A definite improvement over the previous generation's Mario Tennis games.
Compared to the N64 game, the characters aren't as powerful at the net. I'm not so sure about this mechanic, but I guess it means that people who rush to the net aren't able to score easy victories. You aren't quite able to get as much angle on your shots either. This makes the game harder, but at the same time, hitting the ball outside of the court lines is nearly impossible, as are faults on your serve. So basically it has been balanced to make it easier, but more competitive during rallies. Not a bad thing all round on paper - in practice? Only a long time playing Open will tell me whether its for the best.
The gameplay on its own here is superb. Tennis is FUN in a video game when done right - just ask Pong! I don't particularly care for Tennis is "the real world", but its ace here

The gameplay is easy to get into, but really hard to master in Mario Tennis. You really have to watch your opponent and think about positioning him or her, before you really attack a shot and hope for the point. Rallies can get really exciting, and the underlying engine and physics work incredibly well here in Open. The fun of this game, the sheer enjoyment of a great game of Mario Tennis is some of the best fun in gaming - up there with a craxy rampage across Vice City with half an army after you, a kill-spree on Killzone, a stupidly long combo on Street Fighter or a game of GoldenEye 007 with your mates around a telly back in the day.
Overall I would give this game 9/10. I don't think its
quite as good as the N64 game in playability terms - the control system is a tiny bit inferior, as are the shots and balance of the characters. Some of the balancing is a small bit off, like scoring an ace is near impossible for example, unlike in the N64 game. But it is different enough and beautiful enough to play if you have the N64 game, particularly for the online mode, and there are improvements in many areas such as the graphics - plus its portable and easily the best portable Mario Tennis to date. Its not a huge game, but it is a very fun one - you can put it in your 3DS and enjoy it anywhere in short or long bursts whenever you feel like an enjoyable bit of video gaming. Its had some flak but I believe its well worth a shot!