The key is knowing the inside scoop on his knee. When he left Miami, rumors abound that he had a degenerative knee condition. That could have just been talk, typical when a talented guy is leaving a team and everyone involved are trying to feel better about it. But, if it was more than just talk, and if it explains his apparent lack of speed, then you might have to pass.
Otherwise, if that's all just BS, then I'd re-sign him in a heartbeat. It's not like we're low on funds. He's not young, but he's not ancient, either. The Giants just won the Super Bowl with the aid of Sam Madison.