It's perfectly usable for professional work. There was a recent question on the GIMP developer thread asked by the head of a company to find out if GIMP was right for their graphics department. Their artists demanded a bunch of things that they were sure GIMP didn't have, or couldn't do. This was my response, having successfully replaced Photoshop with GIMP quite a few years ago:

3 Single row Marquee tool 4 Single coloumn marquee tool -Draw a rectangle selection across the canvas from end to end where you want the 1px selection. -In the rectangle select dialogue, scroll down to the "size" and enter "1" for the width or height

5 Polygonal lasso tool -The default mode for the lasso tool in GIMP is polygon. Click and drag for mouse mode. -Moreover notice you can move points you have already put down simply by dragging them where you want them (better than Photoshop in this way).

6 Magenetic lasso tool - Try the Scissors Select tool.

7 Quick Selection tool - Try the Foreground Selection Tool

11 Perspective crop tool - Select area to be cropped with lasso tool - Image > Crop to Selection - Use Perspective Tool to adjust

13 Color Sampler tool - Windows > Dockable Dialogues > Colors

14 Ruler tool - Try the Measure tool.

15 Spot healing brush tool - Try the Heal Tool.

13 Patch tool - Filters > Enhance > Heal Selection

14 Content aware move tool - make selection - Copy selection - Filters > Enhance > Heal Selection - Paste selection ... (I have no idea why you'd need a separate tool for this. ;P)

15 Red Eye tool - Filters > Enhance > Red Eye Removal

18 Color replacement tool - Use "Select by Color Tool" - Colors > Color Balance / Hue Saturation / Etc. - Additionally: Colors > Map > Color Exchange

19 Mixer Brush tool - GIMP will acquire brushes that include background color mixing in the 2.9/3.0 release - Check out Krita.org for an illustration program to rival Photoshop or Painter X

21 Pattern stamp tool - Select Clone tool - Scroll down in tool settings to "Source", and choose "pattern"

22 History Brush tool - Open another copy of your current working document from your last save point - select the Clone Tool - In your save point document, ctrl-click on the area to paint from - Switch back to your current document and paint as you like

23 Art History Brush tool - Not implemented, feel free. :) - Check out GMIC. It contains a wealth of interesting painting effects for GIMP.

25 Background eraser tool - Can be achieved with resynthesize as above, but if you must have it in an eraser tool form, feel free to implement.

26 Magic eraser tool - Fuzzy Select tool (Wand icon) - Layer > Mask > Add Layer Mask (check the "Selection" radio button and the "Invert Mask" checkbox and Click "Add")

32 Dodge tool 33 Burn tool - Try the Dodge/Burn Tool. ;P

34 Sponge tool - Try the Blur/Sharpen Tool

36 Free form pen tool - Try the Ink Tool

37 Add anchor point tool 38 Delete anchor point tool 39 Convert point tool

  - All Integrated into the Paths Tool.
  - Check out my 30 second runthrough of the Paths Tool and related functions: https://www.youtube.com/watch?v=3sgI8rCRLbQ

41 Vertical type tool 42 Horizontal type mask tool 43 Vertical type mask tool - Not implemented, these would be neat, but to be honest the Text tool needs a serious overhaul anyway. - Check out Inkscape.org for vertical text, and a much better and more versatile vector text system, and easy masks that don't need separate tools. Turn any text into a mask in two clicks.

44 Path selection tool 45 Direct Selection tool - Windows > Dockable Dialogues > Paths - Click on the first empty box beside your path to show it. It's now selectable with the Paths Tool.

46 Rectangle Tool - Feel free to impliment. This would be useful. - See Inkscape.org for a current solution that works gr...