First of all, using a damage meter isn't all about e-peen. It's also a useful tool to gauge your effectiveness in a team, the results of changes made, and so on. As a software and hardware tester, I can appreciate the usefulness of an accurate test instrument for revealing the truth in matters that are hard to quantify.
Having said that, there appear to be four ways of going about this.
The old standby, DamageMeters, has in many circles lost credibility and is on its second developer (who may no longer be updating it either).
WoW Web Stats (WWS) has the best reputation but the new 2.4 client appears to require a login on thier website, and all I get is a Java exception error and no link to email them about a possible solution, so epic fail is my guess there. I mean, seriously - I see no email link, so the only way to contact the author is on the message board. But the message board logins are closed, too. BLAH! Thank you very little.
(as an aside, I might start taking a hour a night to produce something similar in Python that wouldn't be so effing stupid. I mean seriously, have you ever seen a Java program that you liked to use?)
SW Stats is decent, but buggy, and that makes me a little suspicious (as an example, I can't get pets and owners seperate any more). It usually shows me at the top of the meters, but there is a problem with that which I will address in a moment. Its strong point is that it communicates with other SW Stats installs in the raid and picks up data from other players no matter how far away you are.
Finally, there's Recount, which I was a little nervous about because it, too, is being maintained as a rescue rather having been graciously handed over by the author to another maintainer. Fortunately for a lot of us, WoW addons are source-released. Here's the thing about Recount: in overall DPS, I rank much lower than I do in SW Stats.
I'm running with Recount now because I fear that software that almost always cheerfully reports me in the top slot (and vastly different from WWS reports when I can get them) is not doing a good job. While I like the innovation of IPC between clients, accuracy is better. If I need to improve -- as Recount indicates -- then I'd prefer to know it rather than go through life thinking I was the bee's knees.