I used to have a 290 Crossfire setup. I'd advise against it. Sold off my cards during the summer.
The performance was actually quite awesome. Crossfire today has better driver support than SLI. Less stutter, etc. People who think otherwise are outdated(perceptions change slowly, because it's true that Crossfire used to be awful compared to SLI, but no longer. Hasn't been true since mid-2014 at least, possibly further back).
Still, the reason why I'd advise against is the massive heat output. I was having my own little personal sauna in my living room. It's not a good way to play games. I could only really get it to be comfortable during our winter, but otherwise no.
The 390 doesn't have less power consumption than the 290s, so while I'd say it's a great single dGPU, I would advise against Crossfire.
If you need max performance it's better to get a 980 Ti. If you're fine with a high-end GPU, having a 390/X is great. I'd even say, look at air-cooled Fury(non-X). The Sapphire version is amazingly cool and quiet(check Anandtech's GPU review). It's also more affordable than a 980 Ti.
But if you really need the utmost best, go with a 980 Ti Lightning Edition. It's an amazing card. See TPU's recent review.