Two and a half series is pretty good tenure for a Doctor Who companion - Rose only got two, and Martha and Donna only got one (although they all got cameos in later episodes). Back in the classic era, even the most well-known companions only got relatively short tenures - Sarah Jane Smith and Jo Grant only got three series each, Romana got two and a half series (in two different incarnations - incidentally, RIP Mary Tamm, the first Romana, who passed away recently) and Leela only got a series and a half.
I like Rory's character - he's probably the character who has changed and developed the most in recent Who (closely run with Donna, but she was only in it for one series). The whole "last centurion" storyline made sure of that. It also allowed the Doctor's romantic spotlight to change from the usual regular young female companion to the older and much more mysterious River Song.