I'm currently running the Flux Bitter Sweet plug-in on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I run the plug-in in Pro Tools LE 8 that runs with a Digi 002R audio interface for the most part, but I also run it with a Digidesign Micro Box if I'm on the go. I can't say that I've ever had a problem running this plug-in within my home studio configuration. Then again, I've never needed to run more than two at a time if I can remember correctly. Having said this, I think that most people will find that they don't need to run too many of them at a time either, so as long as you have a decent system where you can currently run some plug-ins smoothly, I think you'll be all set. Either way, it's free so you can at least try it!
You really can't go wrong with a free plug-in! So I can't really say too many bad things about the Flux Bitter Sweet plug-in since indeed a free one. While not the most in depth or powerful tool, it can certainly come in handy on occasion and I'd certainly rather have it than not. It has a realistic tone quality to it with a lot of brilliance, and can definitely add something if you use it right. If you've got a digital audio workstation, I'd definitely recommend downloading the plug-in, as it's free!