At the current difficulty its hardly worth it. For example i had a Rig with 3x ATI 6870 running 24/7 in 2011 when Bitcons were around 25$. I think it made about 20-30 USD a day in profit. The same Rig would make $12 today by running 24/7 even at the current BC price of 218$. Subtracting electricity (expensive in europe) profit is fairly minimal and you have to keep the beast somewhere which is very noisy and pumping out hot air.