EDIT (06/4/11) - I am now editing and considering what I have initially wrote from launch day. My initial review still stands, and it is a brilliant game - offline - however I have noticed some problems. Playing online is very frustrating, gone are the 'tactics' because most of the players who often win are overpowered creations with high stats for their forehand, power and serve. As a result I rarely play online because 'everybody' has the same player types. This results in a 'one strike' tennis on every shot - rarely seen in real tennis with massive flat booming groundstrokes, performing such flat shots in real life is avoided since it causes too many unforced errors (Del Potro, Federer and Soderling have relatively flat forehands, but cautiously use their 'raw maximum power' on shots, instead using a combination of topspin in every shot). Moreover the movement is a little slow for me, reactions and turning on the spot can be quite annoying, for relatively near shots.
This is honestly an amazing game. I play tennis regularly, and understand how strategy and shot making is difficult to execute and the challenges of facing an opponent who plays a certain style. It does this in an accessible and easy to pick up way for newcomers, however provides real depth for more hardcore gamers. Topspin 3 failed doing this, with complicated buttons as well as combinations to play a straight forward shot. Gone are the 'risk' and 'power' shoulder buttons from T3 - instead is 'power' and 'control' are now the shots to achieve. This is done by long pressing a button for a power shot. A control shot is a quick tap of the button for angled/placement shots.
On screen helpers indicate the 'timing' of the shot - crucial for a 'good' or 'perfect' shot. Not doing so will cause unforced errors, bad placements and a weak shot. The on screen helpers are a massive benefit to the game - T3 suffered from a lack of help and support on shots, you simply didn't understand why shots were bad, this is resolved in T4.
Topspin 4 incorporates three 'styles' of play: aggressive basliner, counter-puncher and serve & volleyer. This is incredible important and makes a lot of difference. Playing an opponent who is skilled in either 3, forces you to think how to approach the match and chose your tactics and shots wisely. Fatigue is equally important, do you want to play long rallys against somebody who has high stamina?
I left out lots of things, but understand this is a really good game. It's a 'simulation', however incredible rewarding and accessible for newcomers alike. I didn't like playing Virtua Tennis for the past couple of games since it was too 'easy', no thought was taken towards the strategy of playing tennis (positioning, shot choices) as well as fatigue of players as well as a deep and rewarding gaming system.