What's a good, non-awkward thing to say when being introduced to the significant other of your co-worker/boss when your co-worker/boss has said mainly less-than-nice things about them?

This always come up at our company holiday party; I’ll meet the significant other of my boss or colleague, but never know quite what to say if they have at times expressed mixed feelings about the person, except for “It’s nice to finally meet you! I’ve heard so much about you. ... Only good things, of course!”

For some reason it’s hard for me to spit that out sincerely if I know that’s not really true (even if I try, it comes out machinated); but if I don’t add that last critical component—“only good things, of course!”—they’ll embarrassedly ask it right back. I’m looking for suggestions as to how to handle this situation, because it’s come up a number of times, and I want to be as polite and outgoing as possible. and I don’t really want to piss off my boss exactly

I’m also interested in the inverse of this situation: your significant other is introducing you to his/her employee, and you’ve heard your s.o. complain about their sub-par work for the past year. Is this just one of those grin-and-bear it social scenarios? Have any of you found something else that works?