From what I understand, Jeffords voted with the Democrats a lot anyway. For example, he was one of the leaders in killing the Bush tax plan in its first round through the Senate. So, if he's going to vote that way, he might as well be honest about it. I don't imagine his constituents in Vermont are going to be all that upset, at least on the whole. Look what percentage of them voted for Al Gore.
Where this will have its real effect is in the control of committees. Jeffords will now side with the Democrats in those votes, and the Democrats will start to control the flow of legislation through committee.
I've never been a big fan of the extent of control that the parties had over congressmen. I think they ought to vote with their conscience and not with their party (including in the selection of committee chairman). But that's not going to start happening any time soon, I suppose.