I highly recommend Jonathan Cohn's tribute to Russ Feingold. Over the weekend, I asked liberals who they were especially rooting for tomorrow, and Feingold got lots of mentions. As far as I'm concerned, that's nuts, but I'll have to admit I'm just not a fan. Perhaps its something about that part of the country that I just don't get (I never liked Proxmire, either), but I don't know why liberals would care so much about someone whose main accomplishments were symbolic, and who couldn't be counted on to support his party when it mattered.
All that said: I do think he's been a good Senator, in one important sense. As I've said over and over when talking about the filibuster, United States Senators have strong incentives to create a decentralized, non-hierarchical, body. The problem is that such a Senate doesn't go well with boringly partisan Senators. If we're to have a Senate in which individual Senators matter, then it's not such a bad thing to have some Senators really act as individuals. You don't have to be independent of your party to do that -- Ted Kennedy was as orthodox as they come in his voting record, but he sure wasn't just a generic Democrat -- but it's one way to do it. By that standard, I think Tom Coburn is a good Senator; Bernie Sanders is a good Senator; and Russ Feingold is a good Senator.
I'll add that in a two-party system, it's not such a bad thing that the institution encourages Senators to act apart from their parties. It's a good thing when Americans whose views don't really match up with either mainstream Democrats or mainstream Republicans can point to at least one of a hundred Senators as "theirs." For good government liberals, Russ Feingold has been a match.
It's not the only thing that the Senate needs -- the Senate also needs workhorses, someone like Paul Sarbanes, who was a great workhorse in the Senate, wasn't exactly an "individual" of that type. (You can be both, as the Ted Kennedy example shows). I don't think that's Feingold's thing, though, although he did do serious legislative work on the campaign finance stuff, I suppose. Russ Feingold has been a good Senator in the way that Daniel Patrick Moynihan was a good Senator. As I've said, he's not to my tastes, but I guess that's the point; if we're going to have something like the United States Senate, we should hope that it doesn't get filled with 100 nameless timeservers and party hacks, or else there really is no point to it. And I say that as about the biggest fan of party hacks that you're going to find.