Pretty much like you, I got a 7970, then a 3rd 1080p monitor and then wanted higher fps so got a 2nd 7970. Poor decision
occasionally a game plays well enough to run with decent settings at 5680x1080p and it makes you happy!
Bezel compensation is good!
having to drop image quality with most games to get playable frame rates or having to revert back to single screen for high frame rates with high image quality
poor load sharing between cards
Personally having dabbled with X-Fire (currently have 2x 7970s) and eyefinity (just sold my 24" 1080p screens) I would say don't bother with either. Save yourself the money and headaches and get a decent 27" 1440p IPS monitor (what I've gone with now) and upgrade to next gen 8000 series if you still really need extra performance