The IK Multimedia T-Racks Metering Suite is a set of different meters for mixing and mastering. It can be bought as an individual plug-in but is also found in the T-Racks 3 Mastering Suite, which is where I've used it primarily. I can't speak to the installation process for installing this on it's own, but if you have T-Racks 3, the process of installing that is easy enough and isn't out of the ordinary at all. The different meters that are available here include a peak meter, a perceived loudness meter, RMS, phase, correlation, and a full spectrum graph. All of these are self explanatory in terms of their interfaces, so there is no need for the assistance of a manual.
I've never had a problem running the IK Multimedia T-Racks Metering Suite. I should say that I've never had a need to run it outside of a master channel when I run it in Pro Tools, but most of the time I use it in T-Racks 3. I'm currently running it on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. It's hard to say how well it would run on a given system without trying it, but if you have a system where you're currently able to run your DAW without issues, you should be okay.
In general, I find the IK Multimedia T-Racks Metering Suite to be extremely helpful when it comes to doing home mastering jobs inside and outside of T-Racks 3. The meters that I use most often are the peak, perceived loudness, and the spectrum graph. All of the meters are very helpful, but these are the most useful in my opinion. I'd recommend getting the full T-Racks 3 mastering suite instead of getting this as an individual plug-in, as it's worth it for the extra dough and you'll be getting a whole slew of plug-ins with the full version of T-Rack 3. The T-Racks Metering Suite is probably the best of it's kind that I've used to date...