The result is fascinating, though sometimes over the head of just about anyone who isn't John Carmack. I'll just steal the most interesting bit from day two of the interview, so long as you promise to be good and click the link to go read the rest. Here's The Carmack sizing up the current generation of video cards:
The ideal card for DOOM hasn't shipped yet, but there are a couple good candidates just over the horizon. The existing cards stack up like this:I can't wait to see all that eye candy in action. Mmmm...
Nvidia GeForce: We are using these as our primary development platform. I play some tricks with the register combiners to get a bit better quality than would be possible with a generic dual texture accelerator.
ATI Radeon: All features work properly, but I needed to disable some things in the driver. I will be working with ATI to make sure everything works as well as possible. The third texture unit will allow the general lighting path to operate a bit more efficiently than on a GeForce. Lacking the extra math of the register combiners, the specular highlights don't look as good as on a GeForce.
3DFX Voodoo4/5, S3 Savage4/2000, Matrox G400/450, ATI Rage128, Nvidia TNT: Much of the visual lushness will be missing due to the lack of bump mapping, but the game won't have any gaping holes. Most of these except the V5 probably won't have enough fill-rate to be very enjoyable.
3DFX Voodoo3, S3 Savage3D/MX, Matrox G200, etc: Without a stencil buffer, much of the core capabilities of the renderer are just lost. The game will probably run, but it won't be anything like we intend it to be viewed. Almost certainly not enough fill rate.