I don't remember there being multiple specials. The only one that I know of (day at survival mountain) can be watched almost anytime. I'll probably say after episode 35 (so echizen has a chance to settle into the team) but before episode 70 (around where one of the characters leaves, I have no idea what the episode really is, and I'm not telling which one leaves, its a surprise&#33.

While I will say Ippo has no comparisons (it looks like whatever Ippo-comparison was made was edited out of the original post), I will say that Prince of Tennis is my favorite sports anime right behind Ippo. Unless you consider Inital D a sport anime, in which case its my third favorite sports anime. Unless you consider sexy no jutsu a sport, which makes it fourth...