Comments: I found these balls simply put and more than anything else, don't bounce consistently. (There are other issues with these balls, but their unreliable bounce is what is most important to me.)That might not be a problem if you're a mile behind the baseline and have time to adjust, but, for me, who starts his swing often before the ball hits the court, it's a big deal. I routinely half-volley shots off the baseline, and Penn balls aren't reliable for me.
From:

Comments: I have been playing tennis for the last 20 years. I bought a case of Penn XD tennis balls a few months ago. They were the worst I'd ever played with. They did not bouce well at all. I hate it. Those tennis balls were terrible.
From: Gigi, WI, USA. 05/09