Our drummer uses an eKit, and it's great! When we're playing at volume and everything is belting out to the audience, most people are hard pressed to tell the difference during a live show. A lot of the subtle nuances that are possible with a normal kit are not possible on an eKit. However, most of these will be drowned out in all likelihood during a lively set, so you don't really notice it on the eKit unless we play quieter, but even then it's not massively noticeable.
The big difference for me is the level on stage. Because there is no large kit to amplify against before you even begin to amplify for the venue, the backline is much more pleasurable volume wise, and better for the hearing as a result
You do need a good eKit though, cheap ones just don't cut it.