I am currently running the McDSP Synthesizer One on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I run the software in Pro Tools LE 7.4 that runs with a Digi 002R audio interface. Since installing the McDSP Synthesizer One, I've had no issues at all running a few of these at once if I choose, but I usually just use on at a time and I imagine that is how most people will use it. While it really depends on what machine you are running the soft synth on, it has always run smoothly and stably within my configuration.
I've had the McDSP Synthesizer One for about six months since I bought it with the McDSP Project Pack plug-in suite and I have to say that I'm extremely satisfied with not on the Synthesizer One, but the entire plug-in bundle. Within the bundle, this piece of software is incredibly cheap and the whole bundle is worth every penny, as it contains a number of other valuable plug-ins in addition to the Synthesizer One. The synth sounds you can get with the McDSP Synthesizer One are extremely varied, with the ability to create and store your own sounds, which comes in handy all the time for me. Overall, a great piece of software that I highly recommend as a cheap option for anyone looking for a nice soft synthesizer.