People in comments on Kerry's Vietnam record and the Swift Boat ads have assumed I believe a number of things I don't believe and forced the conversation in directions I wouldn't normally care about, so I wanted to list what I find most important on this issue, in something resembling a decreasing order of importance.
1. It's dangerous to think that a president should have any military experience whatsoever, even in a time of war, though one might think some command experience (whether as president or in some other setting) would be helpful for a war-time president (though experience as president is much more helpful compared to any other experience). It undermines the whole point of having a civilian as commander-in-chief when we place too much emphasis on military service of presidents or presidential candidates except insofar as that service might exemplify character traits, particularly ones that we have further evidence not to have been overcome. Let me repeat this before I go on. None of this stuff should be on the list of major campaign issues.
2. John Kerry made a big deal of his own military service. That doesn't justify anyone who might have lied about that service (update: nor does that lying justify those who might have lied about Bush's; thanks to King of Fools for some of those links), but it does mean there's justification for evaluating his claims. Bush hasn't done any such thing, and therefore there's less point to comparing the two in a way that assumes Bush is making any argument for reelecting him based on how he conducted himself 20 years ago. Kerry is making such an argument. Comparing the two is legitimate, and each's service should be a matter of public record, but no campaign should rely on that service as an argument for why their candidate should be president if for no other reason than the first point above, and those who don't abide by this are simply asking for more attention to it than should be necessary. That doesn't mean the campaign of the other candidate should give it that attention, and it certainly doesn't mean the campaign of the candidate whose service has become a part of his argument that he should be president should then go and give the same kind of attention to the one who never thought his service was a reason to vote for him to begin with.
3. Both candidates probably have things in their past that they regret but are embarassed to focus much on. The difference is that Kerry doesn't know how to stay away from them. I'm not sure why so much attention in this election has been given to the events of 30 years ago when there's much to talk about regarding our current concerns. Those who oppose Bush should focus on why they don't like his presidency and give arguments for why the things he's done are things that shouldn't be done. People have done this, but that kind of discussion seems to have been sidelined lately. Those who oppose Kerry should deal with what he's actually said and whether it supports a new president rather than Bush, whether the things he says anything specific on are importantly different from Bush, and if they are whether they are in the right direction from where Bush is on them or if they're the opposite direction from where we should move on to, as I believe about most issues Kerry is clear on.
4. There are people on both sides who have without sufficient evidence accused the other side of doing things that I have no reason to believe the other side has been behind, and these people are thus guilty of conspiracy theorizing. It's not true, however, that both candidates have questioned the service of the other. The candidate who has made his own service a part of his campaign has questioned the service of the one who has not made his own a part of his campaign. The questioning hasn't gone the other way, not on the level of candidates anyway. There are friendship relations between someone of Bush's people and some of the people who are doing it, and there are cooperative relationships for some events and initiatives between Kerry's people and 527s that also say unsupported things about Bush, but neither campaign has publicly said much officially to condemn the other in these ways (and I say "much" because Kerry has said some things from time to time).
5. Kerry's records go a long way toward questioning the claims of those who attack him on his service, though I won't say that they confirm everything he has said, because there are still some discrepancies and question marks. What I don't see coming out of the major news outlets, including Fox News (not counting partisan debate talk shows like Hannity and Colmes or opinion people like O'Reilly, which aren't news!) is that the same is true of Bush's records. They pretty much confirm everything he's said, with a couple discrepancies and question marks on fairly minor issues (at least minor compared to the kind of charges that get leveled: AWOL and deserter, for instance). There's a good summary of that here. I don't like the tone of this other one in places, but it's also good on information that gets left out all too often. For a wider perspective, these do a good job, and the remaining complaints about Bush are much less important than some people are making them seem if this is right.