i also purchased the steam-version during the winter-sale and experience the same problem.
I´ve overcome the initial crash (right after the beginning dialog) by switching to window-mode. After that, i was able to play about 18 hours (act II) without any crashes; but now the game dies with every transition or save...
I´ve got a Late-2013 MBP (https://support.apple.com/kb/SP690?locale=en_US), 16 GB running OS X 10.10.5
I´m currently downloading the El Capitan-Update, hoping this might solve the issue... these 18 hours really got me hooked
Thanks for your support