I'm not a graphics professional, only a developing artist.  I have Photoshop CS2, Painter IX.5, ArtRage, Twisted Brush, Corel Suite, Illustrator, PSP on my Win XP computer.  I have PS CS3 and ArtRage on my Mac Pro laptop.  I am going to by a Mac Pro desktop next month, and I will use PS CS3 and ArtRage on it, maybe Painter X.

I am a painter, and started with Painter, then I added Photoshop.  Hmmmm!  Sweet!  I found myself using PS more and more.  It was easier to create my own custom brushes.  It was easier to adjust my brush controls.  It gave me solid, stable performance.  Painter was not as stable, and crashed alot.   ArtRage has replace Painter for painterly effects.  It is a secondary program for me.  I have used Twisted Brush also and I like it for some things.

You mention an area I'm not interested in...3D, and while I know you can do some 3D work in PS, I have never tried.

My vector needs are accomplished in PS, so Illustrator wasn't necessary for me.  I never use PSP.   I also never  use Corel Suite. 

My primary program has become Photoshop because of the variety of uses, the stability, the ease and flexibility it provides me.   Of all programs, it would be the first I would buy and did for my Mac.  The more I use it, the less I need other programs.   It is fun discovering all it can do.

My only disappointment is the limited plugins that are available for Mac versions.  However, I am finding I am willing to pay for really great quality plugin that do what I want, rather than having hundreds of plugins with single purpose or one that duplicate others in part.  

The one plugin that I will be beta testing asap on my Mac will be Filter Forge.  I already have it on the PC and love that one.  You can create your own filters and the variety of filters are amazing.

If someone told me I could only have one graphics program, I would choose Photoshop, and if I were unable to get it for some reason, I would go through a severe withdrawal.

“Music is harmony, harmony is perfection, perfection is our dream, and our dream is heaven”
Henri Frederic Amiel