It's quite usable for casual users who don't know the names of the applications and want to explore what's in the computer. KDE offers a gret solution for this; in the start menu applications are organized by category and there's a short description next to the icon explaining what it does.

If you enter a category it doesn't add a layer, it shows the applications in the same view as if you were opening a folder; and at the bottom you always have a Search bar which works great and power users can comfortably use.

So, say, you want an application to play video. You go to Applications, select the Multimedia category, and click on the icon that says "Video player".

Both Unity and Gnome3 also have solutions in which you don't have to compromise or use quirky macros to get the behaviour you want. You can have the best of both worlds. But then, as usual, many will continue bashing Linux GUIs as if they were a usability nightmare and totally inferior. C'est la vie.

There is a lot of truth to that. But I would wager that 99% of the time you know what app you want to open and its just a matter of finding it. Typing a few letters of some part of its name is definitely a win here. Another nice solution would be for apps to have categories that they can install themselves into, and the user could filter by typing generic categories like "internet", "play video", etc.