I didn't have any problems installing the Soundtoys Phase Mistress plug-in as I didn't have any compatibility problems or any other issues. I had the plug-in up and running within a few minutes and since haven't had any problems with it at all. The interface is also quite easy to follow as it has a variety of parameters including knobs for mix, frequency, resonance, modulation, rate, shape, and input and output. There is definitely a lot you can do with it as there is also another modulation screen that you get pretty specific with. I don't believe that a manual is needed because of this straight forward interface, but can't say how well put together the manual is as I have never seen one.
I am currently running the Soundtoys Phase Mistress 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 7.4 that runs with a Digi 002R audio interface or Digidesign Mbox Micro depending on where I am. Within this configuration I am able to run this plug-in without any issues at all as this is a plug-in that doesn't generally take up too much processing power. I have only had a need to run two or three of this within a single session, so can't really speak to how well it would run outside of this limit, but wouldn't think that you would have a problem as long as you have an adequate system.
I've been using the Soundtoys Phase Mistress plug-in for about six months and it is definitely one of the most in depth phaser plug-ins that I have used. The possibilities are almost endless as it gives you a ton of parameters to work with and I find that I am able to get a wide array of different tones. While I don't use this plug-in everyday, I like having it in my collection because often I do find that I would like a phaser effect, and I never know when this will come in handy. If you are looking for a great sounding phaser plug-in that is easy to use and versatile, definitely check out Soundtoys Phase Mistress.