It seems to be the most complex and real ATC client around: Euroscope. The voice of one user:

"Once again I am amazed at the dedication of the members of VATSIM. Many of whom devote there time, expertise, loss of friends wives and sanity for the good of the online simming community. One such individual is Gergely Csernak who through sheer strength of will has developed a Radar Client for use by controllers on VATSIM. What Gergely has created is something that once again moves controlling into a totally new realm and makes "real as it gets" even more real.

Euroscope is truly a masterpiece in the same way that Ross Carlson's VRC is but for very different reason. Eurocope is far too complex to go into any detail on these pages but the one thing I will comment on is the way that as a controller almost all aircraft commands can be made via the tag.

The other revolutionary aspect of Euroscope is the introduction of sectors under identifiable control. Once again difficult to explain but once seen in action is really quite startling. I can only suggest that you have a look and make up your own mind. Eurscope was released over a year ago but after the release of an upgrade that makes it is easier to use I believe many controllers in Europe may just migrate over to it. Competition is good. Thank you Gergely and all your team of helpers and beta testers."

No doubt, Euroscope is the master tool as ATC software in VATSIM. And (like all master tools) it will take time to master this tool. So don't expect a one-click installation until the software rolls out its real performance. You certainly will need training with it, a good time in reading and understanding the concept and usage, but then ES will be your closest approach to real life ATC (unless you are a real life ATCo).

Euroscope and the "power of control" can be found here