E6 usually devved in C41 so they can be printed on colour paper with added contrast - in some situations the resulting prints can look quite good. It depends on what result you want. C41 devved in E6 looks rubish IMHO. You get positives with a browny orange colour cast from the base layer (slide film doesn't have this base layer).

I used to do a lot of cross-processing at college because it gave the results I wanted. Though most labs won't develop your films as some believe the film itself reacts to the chemicals and so contaminates other neg films that go in afterwards. I reckon this is a load of rubbish myself, but you never know.