I've used the Colour version, and have to say it didn't particularly impress me nor did it disappoint. It's better than the liquigel (even in our hard water it would suds like crazy, even the Miele couldn't rinse it out), and Small & Mighty creates little if any suds at all and the first rinse is always clear. However, as with all liquid detergents, it doesn't hack ground-in stains very well. I'm not sure if this was because it's a colour care product or what, but it did leave tinges of brown on jeans which were heavily soiled with dried and caked on mud - rewashing the jeans in powder removed all traces of the mud, so I think for the price it's selling for it is rather overpriced, if it can't perform better than powder which is cheaper by the load. But no doubt as time goes on Small & Mighty liquids will probably drop down in price, and it would also be interesting to see if P&G come out with any alternative products.
Regarding the concentration, I think mrx is probably right - I remember a few years ago liquids did become more concentrated/thicker, so in comparison Persil Small & Mighty is probably only twice concentrated than our previous liquids, whereas the American Small & Mighty liquids would be 3x concentrated as standard American liquids are almost watery in thickness.
At least this is a first sign of concentrated & compact detergents making a return - here's hoping that they bring back Ultra powder next and get rid of those awfully dumbed down (but fun) liquitabs! Worse culprits are washing tablets, although I'm using them at the moment, despite the boxes being smaller and the tabs being effectively concentrated powder, they do have an awful lot of packaging waste!
Jon