As Beowulf says, like anything, you get out what you put in. The more time you spend learning your way round the program, both the easier it will become and the more complicated things you'll be able to do in a shorter time. But you do have to spend time "apprenticing" to it! You're not gonna be turning out the likes of what's posted up on this thread, Mike's and elsewhere overnight; it
will take time to get to grips with it.
With .NIF files you need to be careful; just because Blender can import them doesn't mean you'll be able to export them and use them in TM (they seem to be ultra-compressed for their level of detail); I've had quite a few that are widdly in size as NIFs, but when you've converted them, they end up so big that there's no way in hell I could use them without them crashing my computer. You need to know what mesh pieces you can safely delete to bring the size down to a manageable level, etc, and that mostly comes with trial and error.
Costume modelling is an entirely different kettle of fish, to boot
Machinima maker? Using The Movies? iClone? Moviestorm? Any other engine? Then check out TMUnderground at (http://www.tmunderground.com/)