How is the soundstage on these? I don't know how to gauge semi-open. Will I be able to talk to coworkers without removing these?
Conversely, will I be annoying my coworkers by letting my music leak out if I decide to turn it up a little?
HeeftyYes, you will be able to without any problems. Leakage is not very high and you should definitely prefer them over full-open back designs.
As for soundstage - it is the same as in K240 MKII / Studio models. Overall very good and can compete with some of the premium class headphones. Headroom is more spacious in L-R axis than F-R axis, but that is quite typicall for most of dynamic headphones. And with this size - very rare for price tag this low. You can read more on head-fi in their topic and my full review: http://www.head-fi.org/t/775466/akg-m220-semi-open-studio-headphones/30#post_11945237
Conversely, will I be annoying my coworkers by letting my music leak out if I decide to turn it up a little?