I bought my first 7950 in April 2012 and bought a second one for Crossfire in Feb 2013. Yes there is a noticeable difference, BUT only in game performnce. When doing anything else besides gaming thanks to AMD's power saving features the second card shuts itself completely off.
As far as do you need it, what resolution are you gaming at and how heavy of a gamer are you. If you're a light gamer running 1x 1080p monitor than most likely Crossfire wouldnt give you enough value to justify the cost of another card. On the other hand if you're like me and run something like a 3 screen EyeFinity setup where I will sometimes game at 5760x1080, the boost in performance is pretty much required if I want AA on or to run games anywhere near high or max settings. I'm still waiting on them to fix the runt frame issue in EyeFinity setups, but overall I'm happy with having 2x 7950's. Enough power to game on 3 screens, or if only using one screen I can pretty much max any game out.