I think Agassi handled his retirement perfectly. It was time to go, but he fought until the end.
I think if Roger can stay healthy and continue to play at this level, he might still win one more Slam. It's not like he's really fallen off his game. He's still ranked 4th! Most players would be thrilled to play at the level he's still playing at. Age isn't in his favor, but he's still one of the best out there.
That being said, I think he's probably set for money with all of his endorsements and winnings, so if he wants to retire and spend time with his family, I wouldn't blame him. He's one of the all-time greats of the sport and he doesn't have anything to prove. I definitely can't see him pulling a Connors and staying in the game into his late-30's and becoming a side-show which, sorry, is kind of what Connors turned into.