The TC Helicon Harmony 4 plug-in is an in depth harmonizer for your digital audio workstation. I don't own the plug-in myself, as I have only used it in a professional recording studio, so I can't say anything about the process of installation. However, there definitely were not any compatibility issues, and since I've been using the plug-in myself I haven't had a single problem with it at all. The interface of the software does have a good amount going on, and there is certainly a nice amount of control of your sound that is available here. You've got main parameters to control the mode, one to choose the key and one to choose what type of scale you'd like to use. There is then a graph that helps you to choose the interval voicing of your harmonies, which will give you up to four extra voices. Each voice has parameters to control the voicing, pan, gender, and level, as well as menus with a slew of parameters for harmony, humanize, and set up. To the far right on the plug-in you've got a bypass button as well as a parameter for your wet/dry level and input and output metering. Lastly, there are a plethora of presets on the bottom of your screen to choose from. If you are new to the world of virtual harmonizing, I'd recommend having the manual around to help you get more acquainted with it.
I have run the TC Helicon Harmony 4 plug-in in a Pro Tools HD system only, so of course I haven't had any problems with running it. I can't say how well it would run in an LE system or something comparable, but it isn't the type of plug-in that you need to have on every track. I tend to only use one at a time, which is what I'm sure most other users will be doing as well. Having said this, I'd recommend trying out a demo if they have one available to see if it will run well on your system.
When compared to the other harmonizing plug-ins out there, the TC Helicon Harmony 4 definitely stands its ground. It has a pretty realistic sound quality (at least as realistic as you can get for a fake harmonizer) and certainly has a lot of control over your sounds. In general I'm not a big fan of digital harmonization, as I like to get the most organic sounds possible, but sometimes it can definitely be nice to have - especially for experimentation and thinking outside the box. If you think that you would like to add a harmonizing plug-in to your plug-in suite, I'd encourage you to check out TC Helicon's Harmony 4 in addition to the harmonizing plug-ins made by Eventide and other manufacturers.