Do you use your AmigaOS 3.x, AmigaOS 4, MorphOS or AROS operating system?
MorphOS as my main operating system and AmigaOS as hobby one.
What do you use it for?
For everything I need to do with computers. Internet, music, movies mainly. Classic Amigas for gaming and demos.
At which areas can AmigaOS compete with Windows, Linux or MacOSX today?
MorphOS can compete very well with movie playing

MorphOS version of mplayer just works so well out-of-the-box, that I never have had so easy playback on any platform. Even on Windows players suffer for codec hunting, subtitle problems, audio problems etc. I have noticed it many times with same video files on several platforms. Always problematic on Win, Linux, Mac... even mplayer on those platforms suck with default settings with subtitles etc.
Oh and others don't have any chance on fun and quickness of new amigaish systems

And everything feels so clumsy and restricted compared to MorphOS if you take count window handling etc usage issues.
With OWB for it. Amiga have atleast taken a right step into the others world. But what about all graphis, audio and games etc?
I guess it can't compete with professional use, but for end users' everyday photo manipulation etc there are good programs on Amiga/MorphOS. ShowGirls is excellent tool for digital photos and older Amiga's paint/image manipulation programs work well enough still.
For gaming, the old good games won't disappear anywhere and there's fresh air for them with WHDLoad etc. But new games is a problem. Games are always made for current mainstream platform. It's same for all marginal platforms.