Been using CMC to trade CFDs for two years now.
I only trade large caps.
Techinically, the spread is supposed to be identical to that on SETS, however, recently they have introduced a min spread on a few shares I folow - RBS is an example. Min is now 2p, whereas SETS may often be 1p. A few others as well.
Having said that, 2p on RBS @ 1450 is not too bad. And pleanty of others are SETS spreads.
Have tried GNI demo version - nice prices, the the charges are silly IMHO. (£900 racked up in a day's trading).
Hope this helps.