KaRcop,
C4 Engine's website shows some demos of what the engine can do, The games which use the engine are indie games - Any big game developer would create their own engine or buy another engine rather than using a more general-purpose engine like C4. The best example of the engine I think is the World of Subways game which is very detailed.
There is a thread in the suggestion's sub-forum which is dedicated to engine suggestions. As it goes, the C4 engine is quite capable of doing what the devs want it to do - And they have the source code so they can extend it if they wish to add features.
Also, in terms of "The best engine to date", i'd say the cryengines are far ahead in terms of graphical techniques, and I would argue that the
Infinity Game engine is quite far along in terms of procedural generation, along with the Spore engine. "The best" is something very hard to measure.
