Actually quite good once you get into it
The reason for this is the way you direct your shots around the court. In the more 'arcadey' tennis games, to hit the ball to the left (for example), you simply hold down the left directional button, and no matter how long you hold it there for your player will almost always hit it in. This makes these games easy to pick up and play, but means that anyone can become proficient in almost no time. With Dream Match Tennis, however, shot placement is all based on timing. In this game, to hit 'inside out' shots you have to let the ball come slightly behind you before playing the shot, and to hit it in the other direction you make contact while the ball is in front of you. The earlier or later you play the shot, the more the ball will go in either direction.
Of course, this also means that it is very easy to miss, especially if you're a beginner. Leave the shot far too late and it will go embarrassingly wide, sometimes (in my case) nearly hitting the umpire's chair. Once you get good at it though (like with real tennis), and can hit a perfectly timed backhand winner down the line, it will make the practice worth it (well, almost).
Serving is also similar to this- one thing I didn't like about the Topspin game was that I might hit a 145 mph serve right on the centre line, and my opponent would just casually slice it back into the corner. In Dream Match Tennis, you have to hit the ball at the right time in the service motion to get it in, but if you land a first serve on the line, the likelihood is your opponent will be motionless as it shoots past him.
So, all in all, Dream Match Tennis offers (I think) better gameplay than most of the other tennis games, even if it doesn't have the greatest graphics. Also, the fact that you can buy the whole game off the internet for only a tenner makes it quite a bargain.