I tend to use two, primarily -- RockSim when I'm piddling with a design and want to get CP/CG squared away, first.
Then, if the rocket isn't high-performance, it stays in RockSim. If it's high-performance, I then move over to RASAero for more accurate numbers without the need for lots of manual adjustment of numbers.
My biggest complaint about RockSim? The price is getting out of hand. It seems like every new version, the price goes up another $15 - $20, and now it's up to $120. With a price tag like that, I'd suggest you start by taking a look at OpenRocket.
-Kevin