I personally use neither a Mac, nor iTunes (HATE), nor iOS so FLAC works for me fine, and it's supported in Serato ITCH 2.2.2 and Serato DJ. I'm quite satisfied. I've used FLAC for the better part of a decade in my personal collection without issue.
The advantage FLAC has had over ALAC is that it was open-source, and royalty free. Till 2011, ALAC was not. You can imagine which one the significant majority was quicker to adopt..
FLAC, in and of itself, is technically superior to ALAC, but lacks native support on Apple devices or operating systems. So, if you're tied down to Apple, I suppose ALAC would be the route for you to take. If not, FLAC is far more widely supported by a myriad of devices (though neither format has the support that MP3 carries).
It also sees much higher broader usage than ALAC (2011: www.hydrogenaudio.org, 2009: www.hydrogenaudio.org), but this may be an effect of user adoption, device support, and market share of Apple PC's. I'm sure a user is more likely to use ALAC if they own a Mac as opposed to FLAC.