Wow another mecanum debate!
I have always been skeptical about mecanums but I wasn't solely set against using them until last year 2010 driving at an off-season event and realized how easy it is to push those wheels around the field (robot I was driving was 4wd: two pnuematic, two omni). Ever since realizing how easy it was to push them, I have zero desire to use them* unless there is a game that separates alliances from contacting each other. I could never justify a decision that would leave our robot so helpless under defense.
This year our team built a 6wd plaction robot with the mindset as rookies that we will build a robot that can simply play the game (low row + minibot that averaged a logo and a 1st place minibot at regional level. 12 for 12 deployments and 8 for 8 at STL). We knew we wouldn't be the best in most matches and designed our drivebase to be powerful on the field and pushed around many 6wd/8wd and some swerves too. Not matter how bad your upper assembly turns out, you should always design your drivebase to be one of the strongest on the field.
I have heard a lot of "they can out maneuver" or "strafe around opponents" but whenever I hear that I just want to see someone do it (and bad/stationary defenders don't count).
***no octocanums were mentioned in this post***