I just gave it a whirl, because I wasn't quite sure what I would need this for, having the in game manager.
It's pretty slick! First off, it comes with the few that I already use (so good taste there), and second, it's a great way to keep your 'in game' list cleaned up. If you like to download and try lots of plugins, it's a good way to download a bunch, throw in this external mod manager, activate, try it out in game, if you don't like it, de-activate it out in the mod manager, and it's not in the 'in game' list the next time you run the game (or do a /plugins refresh).
Now to figure out how to automatically update our plugins from this site, and to allow us to grab new plugins through it!