The Elysia mpressor is a unique compression plug-in with a lot of control and a great sound. The plug-in is available in just about all plug-in formats, including in RTAS, VST, AU, and TDM formats for both Windows and Mac. This is modeled after the hardware mpressor from Elysia, and while it isn't a match in terms of sound, it's as close as you're going to come and has the same exact make up. I wasn't involved with installing this plug-in as I've used on at the studio where I work only and don't own it. Even if you haven't used a hardware mpressor, the interface of this plug-in should be easy enough to figure out on your own, but it did take me a little bit of messing with it to get totally familiar with what all of the parameters will do for you. Like the hardware version, this plug-in has traditional compression parameters like threshold, ratio, attack, release, and gain, but also has a single band equalizer with a frequency selection and gain parameter, as well as a parameter for a gain reduction limiter, which is what makes this a unique plug-in. I haven't looked at a manual for this plug-in...
The recording studio that I work at is currently running the Elysia mpressor in our Pro Tools HD based system, which runs with a Mac Pro desk top computer consisting of a 2.6 dual Intel Xeon processor and 10 GB of RAM. We rarely if ever have performance problems, so this configuration is definitely not a good judge of how well this plug-in might run. I'd recommend trying the demo to see how well it will run if you think you've got a borderline system.
If you could only have a single compression plug-in, the Elysia mpressor would definitely not be a bad choice. It's versatile enough where you can use it on almost everything, and the built in EQ is a huge plus for doing simple tone color adjustments. While it has flexibility and options, it's also really easy to use and isn't convoluted at all. The sound is really impressive for a plug-in, but again having used the hardware mpressor, it isn't going to give you the same exact sound as no plug-in will be able to truly emulate hardware. If you're looking for a go to compressor plug-in that is more versatile than most, the Elysia mpressor is definitely one to check out.