I bought most Whizlabs simulators, and I also know Ethuware EJBPlus.
Both contain many wrong answers and many ambiguous ones, and neither of them has a good repetition scheme. They don't support much more than repeating wrong answers, which is very bad from an educational point of view. Obviously, they have been created by IT professionals only, without any consultation of education scientists.
Ethuware has three great advantages: The price, the license (*) and the feature to discuss a single question in the Ethuware forum. In all cases where an answer was wrong I got to the corresponding thread in the forum with one click and saw that it was already known as wrong and explained why, and how it was going to be fixed in the next version.
Maybe the quality of the questions is better in the Whizlabs simulator. The Whizlabs support is the best I have ever seen. The replies to questions are instant and thorough. They care a lot for each single customer.
My conclusion is that there is no really great simulator that I know of, and that you should just not spend too much money.
(*) It is hard to reinstall a Whizlabs simulator after you reinstalled the OS, for example because of a crash, and as good as impossible to reinstall it when you changed certain hardware of your system. [ March 29, 2006: Message edited by: Kai Witte ]