Mac
Can't see them getting much traction until they get serious about OS X. Those of us on Macs (I'm not a fanboi - I need to develop for iOS) are still using the wonderful Lync 2011; the wonder being that this was deemed suitable for release in 2011, and hasn't been fixed yet.
There may not be huge number of Mac users, but lots of the decision makers love there shiny Macs, and a business solution kind of has to reach everyone.
Biting the hand that feeds IT
