A search of the forums should give you more details, but I’ll offer my input, based on my personal use.
Q1) Yes, you can paint, sculpt, import & export in ZB3.12 for Mac. A very useful tool, “Subtoolmaster” is still being developed as well as another one “Zapplink”. I don’t deem either critical, but they are useful features. A critical feature missing right now is the ability to directly export displacement maps from ZB3.12. Normal maps seem OK, but displacement maps (especially for micro-poly displacements in other packages) is definitely broken. For me, this has not been a big deal because I generate my maps from object files in other tools (such as xNormal, for example); for others, this is deemed as an unacceptable problem. You’ll have to make your own call on that. 
Q2) I work exclusively with Macs, though I have a Dell PC at the shop that is never on anymore. I keep Parallels 4.0 and a copy of Windows XP running on my systems. Others use Bootcamp (to run things like Softimage). VMware is another good alternative, but right now parallels has the edge in 3D graphics support. (Their site will show you all the apps like Rhino, ZB, etc. they have certified for their emulator.) Performance is not as good as a dedicated PC except in Bootcamp, but it isn’t bad either for the occasional use I need a PC-only application. It’s a personal preference, but there are successful artists with either platform.
Q3) For me, nothing. I just work around a number of the “lumpy” elements in ZB 3.12. ZB has been a cutting edge app in many ways for a long time and you just get used to getting cut yourself every now and again. I have use most of the tools on the market, including Modo, Mudbox, 3DCoat, etc. and so far ZB is the best sculpting solution for my needs, despite some of its shortcomings. I use Modo heavily for creating and retouching base meshes, and building UV maps. I use ZB for sculpting. I use C4D for scene making and animating. I like Maya, but it’s too pricy IMHO for the full kit. XSI is fun, but it pretty much is a Bootcamp app, so it’s not as handy.
All-in-all, I use ZB for its sculpting. It’s renderer is OK, but I can do more in an outside package with materials and lights. IMHO, you’ll need a good Polygon modeler to complement ZB for doing mesh work beyond what zSpheres and ZB can do. This can be anything from Wingz to Silo to Modo, depending on your needs.
All of the above is solely my experience. YMMV,
-K