As someone who has watched Feingold 'up close and personal' from here in his own state I can predict that
1) if you nominate him, he will run a tremendous campaign
2) if you elect him, he won't do what you expect

He's personable, perhaps even charismatic, intelligent without coming off as condescending, quick-witted, extremely independant, and extremely liberal.
A true maverick (and one blessed with horse sense rather than a horse face) I think he'd nonetheless be a hard sell nationally at this point in his career. He's quite far left at a time when the DNC is talking about moving more to the center to recapture/retain republican-leaning voters, and he's from a small state (electorially) that's traditionally voted democratic anyway.
While it's important to remember almost no one gave him a snowball's chance of winning the senate seat he now holds back when he first ran for it, still I'd have to say it's more likely to see him in the #2 slot, if at all on the '08 ticket.
If you like the traditional Democratic party line, you could do a whole lot worse than this guy.