My $0.02....I have owned both headphones and, as always, both cans have their strengths and weaknesses. IMHO, each is build for a slightly different purpose.
Senn HD280 - To my ear, these have more transparent and balanced sound (probably where "boring" or "no personality" comes from). If the recording is badly mastered or of poorer file quality, these are not the best headphone. They were meant for more for pure studio monitoring purposes where you can hear details of music than rocking out and pure music enjoyment. These do have bass, but it is presented in a nature that makes one "aware" of it rather than "feel" it (going back to monitoring purpose). I also found no issues with their comfort; I am 6'4 220 lbs with fairly large head and worn these for 5 hours straight on cross-country flights. Isolation is excellent and I have no problem driving these from my laptop or mp3 player. I would use with with classical, vocal, string, folk, etc type genres (in comparison with M50s).
AT M50 - In comparison to HD280s, these are much more of a "fun" headphone. They are much more immersing in their sound and are not as harsh on lower quality or poorly mastered recording. For many softer and more intricate music genres, M50s had way too much bass for me. Isolation and comfort is almost identical, though I do prefer HD280 pads over M50s. Laptops and mp3 players drive M50s fairly easy as well, but these cans benefit more from amplification than HD280s. I would use these for pop, hip-hop, dance, electronic, etc type genres.
I have owned HD280s for about 10 years...M50 for about a year.