Well, if Stress wants the Borg to be awesome, let no man call for Nerfing!
Seriously though, you only have about 20 minutes to start attacking them. On a large map, it's guaranteed death. You probably only stand a chance on a smaller one where you can get to them quickly, or with at least 8 opponents.
I wrote this somewhere earlier, more or less... you can give yourself more time, but you have to be really stealthy. Romulans obviously work the best for this, but once the Klingons get a cloak they're also great... you need to disrupt the Borg, keep them off-balance. Don't let them build a fleet.
Wanna really screw them up? Sneak a small strike force of cloaked ships into the borg system, if possible a few planets away from whatever fleet they might be massing at the time, which they usually are. Make sure you have ships with enough antimatter to stay cloaked a bit, find a system with a combat lab and take it out... then, if possible, do it again, and then get the hell out of there.
Borg labs are really expensive, and take a while to build. If they've gone and blown most of their resources building ships, which hopefully they have, then you just bought yourself another half hour or so game time before they can muster the resorces to rebuild the labs and then pick up where they left off.
Hit them again before this happens, over and over, and you may have a chance.
If you have 5 or 6 teams agains the borg, than a couple shouldn't focus so much on the actual Borg fleet, they should focus on crippling the borgs ability to build that fleet.