I played for about 4 hours yesterday and thought I'd share some impressions.
Let me start by saying I am a very big fan of tennis games with arcadey elements, with my most recent love being Hot Shots Tennis on PSP (which I have played for about 30 hours). I'm also a pretty big fan of Mario sports games so Mario Tennis Open was always on my radar, although I will admit that my interest diminished somewhat over time as the feature set was revealed and the reviews started trickling in. Nonetheless, I couldn't resist picking the game up to accompany me on a fairly long ride home yesterday and now I find myself here.
I'm primarily in this for the SP and I haven't touched MP yet so my impressions will only touch on that aspect of the game.
From the get-go things are a bit sparse, as your only options in SP are tournaments, exhibition matches, and 4 different mini-games. Your enjoyment of the game over a longer period of time (at least with the SP) will depend almost entirely on how interested you are in acquiring new items to equip your Mii with. You unlock these items by playing through the tournaments but you can't actually use them until you purchase them with coins which are earned by playing the mini-games. It didn't take long for me to play each of the mini-games and get a good feel for what each one entailed, and I can already tell that it will definitely require a good bit of grinding if you want to unlock all of the items. All of the mini-games are enjoyable in their own way but they aren't particularly compelling, I really just see them as a means to an end at this point (must get more coins!).
The gameplay feels solid but not quite as nuanced as something like Hot Shots Tennis. This is particularly noticeable when aiming shots, as it feels like the shot can really only be aimed in three different directions (left, straight, or right). In other tennis games you often have the ability to aim not only left to right but also front to back, so you can attempt to place shots in the front, middle, or back areas of the court. Timing is also rather unimportant since you can just hold down the shot button to charge your shot and your character will automatically swing at the right time. It's nothing new for Mario Tennis but I miss not having the level of detail in the aiming present in some other tennis games.
The Chance Shots mechanic has really dominated the gameplay for me so far as it seems like it is the primary way to win a point. Basically a colored circle will appear on the court requiring you to press the corresponding button (blue for slice, red for topspin, etc.). If you get this right it will cause the shot to be powered up in some way, which will often lead to a weak return by your opponent that opens up an opportunity for a finishing smash. As I progressed through the tournaments my opponents became more competent at returning these shots and executing their own which helped keep things interesting, although I'm still unsure as to how things will flow later on in the most difficult tournaments.
Speaking of the tournaments, they are basically as simple as they come. Each tournament is set up with a basic single-elimination bracket that requires defeating three different opponents to win. There is no ranking system like Hot Shots Tennis or Mario Kart 7 so once you've finished a tournament there isn't much incentive to go back unless you simply want more of the core gameplay (which is honestly still a decent draw to me). The tournaments take place across a variety of different courts but the differences aren't very noticeable and none of the courts I've played on so far have anything in the way of interesting wrinkles or gimmicks (which isn't necessarily a bad thing depending on what you're interested in).
If you don't want to play primarily as your Mii in this game then you will probably be disappointed with the way progression is handled. I certainly wasn't keen on the idea at first and went in with the mindset that I would stick with Luigi but it only took a few matches to break me down and compel me to switch over to my Mii. The reason for this is simple --- unlocked items can only be equipped to your Mii, which basically means that you will only be able to improve the skills of your character if you are using a Mii. This was disappointing at first but I came to terms with it after a few matches and now I am enjoying playing as my Mii and unlocking and equipping new items for him. It also seems like it would be essential at some point since it becomes somewhat difficult to win points without the added benifits of good equipment, although I haven't played long enough to confirm this one way or the other.
Overall I would say I'm fairly happy with Mario Tennis Open but I definitely feel like there are a lot of missed opportunities. I will probably put a lot of time into this game since I enjoy the core gameplay and I like unlocking and purchasing new items but if that doesn't sound interesting to you then I would suggest passing on this one (unless you're in it for the MP, perhaps).