I don't use 3D view but I've used Jesse Rudderman's topo bookmarklet[1], which is conceptually very similar, for years. It's more useful in IE so you can see what tag IE is failing to close but it's useful for getting a quick feel for unfamiliar markup when doing Greasmonkey hacks and the like.
I find it super useful to get a quick insight into a random page. As I'm still learning a lot about HTML, I can go to a website and see a quick visual breakdown of how the elements are composed together.
I use it once in awhile, mainly to get a quick conceptual idea of how a website is layed out. Is it super useful? Probably not, but I think it's still a cool and somewhat worthwhile feature.
I used a few times to figure out why something would work in every browser but IE due to how IE handles event bubble different in DIV hierarchy. 3D view really helped in that regards.
I've played with it but haven't found any great use case (except, now, this). Next time I come across an annoying z-index bug I'll probably bust it out. Anyone who has had the displeasure of build modals on modals (always a good UX) could sympathize.
A friend of mine doing UX stuff actually uses it regularly to instantly see the structure of pages. He's doesn't usually touch developer tools generally, and stays away from any CSS or HTML learning intentionally. But he uses this.
Ha ha .. Geeks and their works. :). I have used 3d view a few times in the past for simple web pages, but as a CAD programmer I'm quite impressed with the rendering performance with this building you just created.
Also using XP here. Make sure you're on the right bar (Firefox has way too many disparate developer tools). It's Ctrl+Shift+I to bring up the Inspector. There should be a button labeled "3D View" on the far right, next to "Style". I don't think it requires any sort of hardware acceleration, the computer I'm on right now isn't exactly recent...
