I've heard the 690 is better than the Titan. From reviews of the 7990, it's still not as good as the 690 or the Titan. I've heard that the only good thing about the 7990 is the bundled games it comes with (around $300). I'd suggest the 690 for performance, but if you want the games I'd go with the 7990.
The 690 has higher FPS, the Titan has lower frame variance. The 7990, for some reason, has been marketed against the Titan a lot of places, when in reality it is a 690 competitor, as they are both top in dual GPU cards, unlike the Titan. Due to driver issues, the 7990 isn't as good.
Yeah, sometimes the 7990 won, sometimes it lost to both. It let me down a little to be honest. It's still the best value because it comes with $300 worth of games. Just look up the review on tomshardware. Between the 690 and titan, the 690 is stronger. But you still have to deal with the issues of sli with the 690, so there is that. It also used more power because it is two card. I personally think the 690 is a better value though.