These OSs are excellent for developers and true workstation use. Some applications have been made available from both Sun and SGI for Linux. Bad thing is that there are no ports for Mac. You can use Windows, Amiga, VAX, MAME, Playstation, SNES, the list goes on. Emulators tend to be slow - don't expect a performance increase from using an emulator.
Many emulators are also a bit buggy.
This seems like it could be exciting as there will be freely available applications to run on it. I use the Window Maker window manager which is based on NeXT Step, and I find the interface very easy and useful. Bringing this sort of functionality and ease of use to the general public, I feel, can bring Apple back up to the large marketshare it had before Windows 3.1. Linux is freely available, it's extremely stable, it's extremely secure, files can be exchanged regardless of hardware, there's the readily available support of thousands of developers around the world, and online resources for anything you may need to do with it. That someone would pay for an OS and applications that crash often, limit choices, is exceedingly prone to viruses, makes ill use of CPU resources is just plain silly to me.