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 great.

47 Rounded Rectangle tool
- If you impliment this, please make it part of the rectangle tool. There is no point at all to having separate tools. :P
- Inkscape.org again :) It's like Adobe Illustrator.

48 Ellipse tool
- Feel free to impliment
49 Polygon tool
- Paths tool already does this. I don't know why Adobe made a separate tool for this. It's unnecessary clutter. :)

50 Line tool
- Paths tool... (hit b to end your line and start a new separate one)

51 Custom shape tool
- Would be cool to see in GIMP
- Check out the Polygon and Star Tool (Inkscape.org)

53 Rotate view tool
- View > Rotate (also Ctrl + Shift + Middle Mouse Button)

58 Masking
- Really? Gimp has more Masking options than the Carnival of Venice. ;P
- Check out Quick Mask and Layer > Mask > Add Layer Mask
- Also check out Mask to Selection, and Selction to Path.

59 Mode change in to CMYK
- Feel free to try to impliment, but read this first: https://wiki.archlinux.org/index.php/CMYK_support_in_The_GIM...
- Check out Scribus for conversion into CMYK (FOGRA 27 is the default, but you can add your own color profiles if that's not working for you)

Hope it helps. :)

-C