The good news is that the Disgaea: Digital Deluxe Dood Edition is now available on Steam along with an upgraded UI, improved graphics, full keyboard/mouse and controller support as well as all of the content the special edition came with.

The bad news is that according to some user reviews and Steam comments Disgaea is currently "completely broken" for some configurations due to horrific FPS drops. Here's the trailer and a bit more information about the whole problem:

After doing a bit of browsing through the Steam forums it appears that not having a controller plugged in might be resulting in these frame drops as the game constantly hogs the CPU in a vain effort to find something that simply isn't there. In these cases just plugging in a controller will bring the FPS back to normal and you're free to clobber enemies with a giant zombie dong. While effective its not the most ideal of solutions given that not everyone owns a controller.

What you can do to protect yourself is to simply buy Disgaea on Steam and then play it for an hour or so in order to see if any lag or FPS issues appear. If they do you can play it safe and simply refund it (which is an easy procedure as long as you've played under 2 hours) or wait for the developers to patch the issue out which shouldn't take long given the gravity of the situation.

Most importantly, don't let these issues discourage you from giving Disgaea a try as it really is one of the most wild and interesting RPGs out there and to miss it would be a damn shame.