... can be found here.

What exactly makes this preferable to playing the scenarios in their native habitats? (ie, their original games)

2 words: Coaster Cam! Thanks! I was just about to ask around for these. :D

Yes, as tinyE says, and I just generally prefer a 3d engine and nicer graphics.

Before I start downloading and installing these, have you used any of them before? The add-on stuff from #1 to #2 was easy to add and worked flawlessly, but not all of these are quite so nice, or even worth the effort so I thought I'd ask.

I have installed nearly all of them (excluding alternate versions) and most are done pretty well, but as you note, not all of them are perfect. I'd say they are worth installing and if there are some you decide you don't like you can remove those. Also, some scenarios have versions from more than one author.