At Exact EPIC I beleive they listed approx. 400 sites own ES and about 135 are currently live.
I would agree with most of what crystalreporting stated, and add that there are still fundamental things that are buggy to date. With all of that, ES does have some really compelling things going for it. The GL data contains full detail from posting packages. I originally thought that the Financials was something to skip in a sales demo, now it we use it as a center point. In an account like AR in the GL key in a customer# and see that customers AR balance. Or print financials and specify a customer or item, and see the impact for that one object on your business. Pretty cool and not many other packages go that deep. Then throw e-Synergy in, and ES looks pretty good.