Anybody using a DVD scaler?

Yes, I use one although I can't be of much help explaining what they do and how they do it, like many others here can. They do improve picture resolution and are worth the money, IMHO. I recently sent mine off to Tube Research Lab for mods ($550) and they brought it to another level altogether; Better focus, better motion, better white level, better sharpness and detail.

Maybe someone can link a web-site that explains in detail the difference between scalers and line doublers/quadruplers? I know I'd like to know more about them.

I don't have one. But I've heard the DVDO stuff is generally pretty good for the money. Most people who have it have or plan to have SDI mods I would assume.

I'm using a Iscan HD processor between my source and
front projector. Since my DVD player offers only a 480i
signal, it made a significant improvement. Even when I
plugged in my progressive player, the signal was cleaner
and sharper via the Iscan processor. THEY WORK, do they
improve by a factor of 40 to 50% as indicated by upscaling
to a 720 resolution, I say no. However, I can comfortably
suggest a 10 to 15% overall improvement.
As Howie said, mine came with a SDI card, of which I'm
currently not using. Seems more of a future modification that I will play with once I buy a SDI source.

Good Luck