I picked up a pair of the Senn 280s and they're very decent headphones. They sounded nasty right out of the box so I put them on a CD player set to repeat and ran them for 48hrs. straight. I take them on location when recording with my Titanium and a Digi 001 because they really do seal you off from ambient sound.
Julian over hear the Senns are half the price of the Beyers. Interestingly Senn has 2 DT-250 models over here. The DT-250-80 and DT-250-250, I have no idea what the difference is and I can't find any info on them.
I've got several pairs of different Beyers and they're all great headphones for one thing or another, but they all sound very different from each other. The DT-931s, their top of the line open phones, are very bright and not anything I'd ever use when accuracy was a consideration. The DT-990 pro model is just the other way around, too much base and very rolled off. I find it very odd that they range in sound so dramatically. I could see a company having a basic sound and the quailty improving as the product line increases in price, but that's not what's going on with Beyer phones.