I bought a pair of mkii's a few months back and have been pretty thrilled with their performance thus far. They play most music well straight out of my computer's headphone jack. Which is saying a lot, considering my sound card is some (likely horrific) creative proprietary POS. They struggle occasionally to render the high ends in classical music, but I am more than confident that such symptoms will rapidly vanish when it is plugged into a suitable amp (just ordered the nuforce uDAC based on Larry's raving review. Hoping it has enough juice to drive them. He said it worked with his Senn HD600s, which are higher impedance than the 271 mkii, so I am hopeful). They do seem slightly strenuous to listen too at times, which I believe is due to lack of proper amplification. The sound that comes out of these cans is remarkably detailed though. Particularly things like gongs or symbols will leave you with your mouth agape. The vocal renderings are fantastic as well. As they are my first set of audiophile cans, I can't make a comparison to any of the senns. I would imagine that both will have a similarly detailed and analytical rendering of music, but suspect that the AKG will exceed the senn greatly in the mids.
Overall, I am exceedingly happy with my 271 mkii. I bought it to replace a pair of $60 Koss noise canceling headphones, and the difference is dramatic. The detailed rendering of the 271 makes music listening a truly engrossing experience. I find myself for the first time wandering around between instruments in songs. The detail is so phenomenal that it is trivial to pick out individual instruments even in massive orchestral scores. I suspect that with a high quality amp and source, these phones can go quite far in terms of extremely accurate music representation. I think the burn-in did them a bit of good as well. They are comfortable to wear, even for extended periods of time. They do exert a slight bit of pressure under the ear, but this dissipates with time as the pads conform to your facial architecture. For the $150 or so price that you can often find on amazon (50% off), I think these cans are almost unbeatable.
EDIT: As some headfiers have said above, the isolation on these is phenomenal. As a college student who uses these phones to tune out distractions while I am studying, isolation was one of the factors I considered, and they have exceeded my expectations. Listening to these at any moderate level, it is absolutely impossible to hear normal level conversation even a few feet away (This is using the velour pads, I have heard the leatherettes are even better at isolation). I don't mean that it is impossible to understand conversation--rather it is impossible to discern that a conversation is occurring whatsoever.