If a divided government is more constrained than a regime where the same party controls both the legislature and the executive, it's also true that the combination of donkey president with elephant House seems preferable to the combination of elephant president with donkey House. I certainly preferred living under Clinton plus a Republican Congress to living under Bush Sr. plus a Democratic Congress.
The economist Jeffrey Frankel has made a good case that within the executive branch, "The Republicans have become the party of fiscal irresponsibility, trade restriction, big government, and failing-grade microeconomics. Surprisingly, Democratic presidents have -- relatively speaking -- become the agents of fiscal responsibility, free trade, competitive markets, and good textbook microeconomics." The flipside is that, in the '90s, the Congressional Republicans somehow became the party of peace and civil liberties while the Democrats went AWOL on both fronts. (If you were a lefty who hated war or had a fear of federal jackboots, your candidate in 2000 wasn't Gore. It was Nader.) Aside from medical marijuana (which has bipartisan appeal) and the drug laws' racially disproportionate effects (most loudly denounced by the Congressional Black Caucus), there even seems to be more room for drug war dissent among elected Republicans than elected Democrats, though it remains a minority position within each party.
With Bush and Ashcroft in power, this trend has been partially reversed: formerly populist Republicans roll along with the administration's most disturbing requests, while the Democratic rank and file rediscovers its anti-authoritarian principles. The Democratic leaders remain as spineless as ever, though, so this isn't much of a trade-off. (Dean, to his credit, is a partial exception.)
Given all that, and given how bad Bush has been, I'm inclined to cheer for the Democrat next year. Some Dems are better than others, of course, and for all the problems I have with Dean I'd find it a lot easier to root for him than for, say, Lieberman or Clark.
Needless to say, the fact that I could cheer for a Democrat does not mean I would actually vote for him. The chances of one ballot making a difference in a national election are almost impossibly small, and if the outcome ever did come down to my one vote you can be sure the results would be decided in court instead of the polls. So with nothing riding on my ballot, I'd rather not throw it away on a man who's sure to upset me once he's in office. Better to back a third party, to write in a cartoon character, or not to cast a vote at all.