I'm using it now for a project that's due to go live early next year and so far it's been a dream. Case in point, I ran in to a bug on Monday and it turned out

I'm using it now for a project that's due to go live early next year and so far it's been a dream. Case in point, I ran in to a bug on Monday and it turned out to be with engine.io itself rather than Primus. The workaround was a one line code change swapping the transformer to something else. Arnout Kazemier (the lead dev) is a really nice guy and when I pinged him about the egine.io bug, he pushed out a change to Primus 24 hours later that worked around the bug and let me continue to use engine.io as the transformer. YMMV but so far I've no regrets using it.