I'm a 4.5 NTRP player and I really like to hit the cover off tennis balls. That said, I obviously needed a ball that could keep up with the harsh pounding a polyester string and cutting topspin strokes would dish out. The Dunlop Grand Prix balls were a worthy contender.
At first, the ball just looks a little different. It is an Optic Yellow color, which is bright and easily seen. Then again, most balls are bright and easily seen anyways, so its kind of a push. Upon hitting with these balls, two things became evident. They felt 'heavier' than the Wilson US Open and Championship offerings I've so commonly used before. You really do need to take a full swing at this ball, even though its the same static weight and size as a standardized tennis ball should be. It's a good ball to hit with, as you take longer swings and the weight makes hitting the sweetspot more pleasant.
The felt is nice, however it does fluff up a little bit faster than the Wilson US Open balls, about the same as the Championships. However, this does little to affect its feel and pace. The 'fluff' quickly goes away and the ball feels solid. The pressure is kept up much longer than any of the Wilson offerings, as they can last a few solid hitting sessions without being a liability. They will not stay as aesthetically pleasing as the US Opens, but they'll keep their bounce longer.
Overall, at the same price, the Dunlop Grand Prix balls are a great choice for a solid set of balls to hit with. The Wilson US Opens may be a better choice for match play, as they are a much more familiar ball, and keep their felt and looks better over a short period, but the Dunlops will be a better set in the longer term.