I am running the McDSP Revolver reverb plug-in on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I use the plug-in in Pro Tools LE 7.4 that runs off of a Digi 002R audio interface. I haven't had any problems running this plug-in with this configuration, as it has always run smoothly. I have never run more than one of these at once because I put it on an auxiliary track and send the signal from the tracks via buses. This saves processing power and I do it with all reverbs and delays. This being said, I don't think you would have too much trouble running a few of these at once if you wanted, but it really depends on what kind of machine you are running this on.
I've been using the McDSP Revolver reverb plug-in for about six months. The plug-in is easy to use and has some great sounding reverb and delay effects. It has a lot of parameters that make it capable of getting a variety of different sounds, and being that I bought the plug-in in a bundle, it was dirt cheap. I love the sounds I get from this plug-in and I highly recommend it for all digital audio workspaces. McDSP makes some great plug-ins for extremely reasonable prices and would recommend looking into the Project Studio LE bundle as a whole.