I've been using Euro Asststant Pro for quite some time now (more than a year).
My opinion is that it's very simple and fast. I'm using Butler (a free program) and using Butler, I have put Euro Assistant Pro's application icon up in the menu bar for fast access (there are other ways for doing this, sure, but i can recommend this option).
Compared to Apples currency conversion in Calculator.app, this is much faster (as in really much faster). Using Calculator.app, you have to go through some time unnecessary steps. Ok, it's not that you have to spend a whole day, checking currencies in Calculator, but Euro Assistant Pro makes it so much simpler.
For fans of simplicity and accessibility, I can deeply recommend Euro Assistant Pro.
For those that are on a tight budget, continue to use Apple Calculator's built-in currency converter, or any of the other free alternatives.
Although, to my knowledge and in my opinion, this is the best currency converter out there. Buy.