I am running the McDSP ML4000 limiter plug-in in Pro Tools LE 7.4. I am running Pro Tools on a Mac Book Pro with a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM and a Digi 002R audio interface. I am able to run a few of these at once if I choose to, and it has always run smoothly and stable with this configuration. While you can run a few of these at once, it is more of a mastering plug-in and is meant to be used on the master fader.
I've had the McDSP ML4000 for about six months and have found it a useful plug-in overall. The plug-in is easy to use as it has four knobs, one of each controls ceiling, threshold, knee, and release. The metering is great and monitors the input level, the output level, and gain reduction. The sound quality is clean and is easy to get a quick, effective level from this. Since I got this within a bundle, it was dirt cheap. I have come to love the McDSP suite of plug-ins as they are reasonable in price and all have a great sound to them. I would recommend this plug-in as well as the entire McDSP Project Studio LE bundle.