I use RockSim on the Mac and would NOT recommend paying for it. I have not used it on Windows, but on the Mac the program will very often (more often than not) not accept input from a mouse click or from the keyboard without first hitting the Tab key. It is just a simple case of lazy programming (how big can the Mac market be anyway). I have gotten to the point where I instinctively just hit Tab every time I hear the Boop-of-Denial. I do use RockSim to simulate the flight characteristics and to "design" some of the simpler rockets. For complex designs I do the engineering using AutoCAD. I did my upscale of the Black Star Voyager in AutoCAD and and am currently working on a 1/6th scale Nike Hercules with it. I use AutoCAD to develop the parts and how they interact. Then I use the CAD "drawing" of the individual parts in their fabrication. I have used OpenRocket. It seems to be a very capable program for very rudimentary rockets (fins, an airframe, and a nosecone). I've compared the simulated Cp and overall performance between OpenRocket and RockSim and they seem very close - with both doing very well in predicting the actual performance (if you get the final mass and surface smoothness correct (Cd)).
Those are the most common, but there are a couple of other simulators that are tailored to the "expert". I have downloaded a couple of them but I do not use them. As is always the case, you need to decide what you want the program to do for you and then find one that does that. Start with OpenRocket and chances are you won't need much else.
Best of Luck!