"I spend about 90% of my time in Lightroom and only 10% in Photoshop."
This is why he was able to do this. He isn't using the functionality that Photoshop provides, he's using the functionality that Photoshop Essentials provides. Look at his parity instructions - every feature is found in Photoshop Essentials. If that's all you use, then great, by all means switch over to Gimp. The bigger the userbase it has, the better off it will be.
But don't think that it's going to replace Photoshop in the near future. Non-destructive editing is the biggest thing GIMP is missing in my opinion, but supposedly the move to GEGL will allow them to start development on this ( https://mail.gnome.org/archives/gimp-user-list/2013-December... ).
Smart objects are another huge feature it's missing - not only the ability to downscale losslessly, but also the ability to edit all replicates at once. This is huge for designers. If I'm defining a user control for a design mockup, and I need to make a change to that control, in Photoshop all I have to do is edit the singular smart object source. The source edits will propagate to all the copies automatically. In Gimp I have to do this by hand. There are some minor features too that bug me. The inability to add a mask to layer groups is a big one for me. Layer effects (while often overused and gaudy) can be really helpful for design work - need to change the color of an icon that's raster art? Just drop a color overlay on it. If you have style swatches, it can be really easy to do fast mockups using this. This in conjunction with Layer Comps (also something missing in Gimp right now) can really help in switching between two or more alts. A great way to see what Gimp is currently missing in comparison to Photoshop is to look at the development roadmap ( http://wiki.gimp.org/index.php/Roadmap ). If some of your heavily used features are on that list, it might not be worth switching over to Gimp. If you don't see yourself as a user of those features though, give it a shot.