I've been proven wrong in the past for doubting Hearst due to his injuries and age - but that was when he was on a good team.
From a fantasy perspective, I wouldn't want Hearst as anything more than a #4 RB, although he may be a suitable #3 in certain circumstances. Not only would he likely have to deal with a RBBC (assuming he goes to the Lions), but the Lions likely won't have many opportunities to run the ball as they will probably be behind by the time the second half rolls around in most games.
As to how he will actually affect the Lions, I think it is a good move for them. I would rather see them sign Hearst than bring James Stewart back - he's better. He should also come relatively cheap, will be a good influence for Artose Pinner and can help bring him along slowly - he's also proven that he can fit in with Mooch's system. However, I would rather see the Lions (or Patriots, Cowboys, etc.) go after Dominic Rhodes. He's young, has talent, and shouldn't cost much, either. ...But what do I know?