I went from crossfired 5870s to a single 7970 at launch, so might be able to help with this question.
For running games at 1920x1200, there is no difference at all. I lost a couple of FPS but got those back and gained a couple more by simply pushing the sliders to max in CCC. The 5870s were already running pretty much every game I had at maximum settings, so does the 7970. It is a lot quieter and I'm using 100W less at the wall, but I wouldn't say these changes are worth the £240 upgrade cost.
When playing at 5760x1080, oh my word, what a difference. It not only allows higher settingsthan the 5870s, but I can also add AA without performance taking a huge hit. Crysis on Eyefinity with everything maxed (inc 8xAA) benched at 1 frame every 4 seconds before, now I'm on 30FPS (might have been 20, but think it was 30). Saints Row 3 needed no AA before, now it runs well with maximum AA. The 7970 doesn't appear to slow down much, no matter what resolution increase you ask of it.
I loved crossfire and had no issues with it, but I have found 40FPS with 2 cards "feels" the same as 30FPS with one. Couple that with the fact that at low res the setups get the same score and the change did feel like an improvement to me, but is the change worth £200-£250? If you're running a single screen at 1080 or 1200, no not worth the cost, but running higher than that or multi-screen, then I'd say it's definitely worth the cost.