So, while I am a first time user of this game too, I think there is need to have some updated documentation about installation. I have it apparently working and did the following to get it from download to running.
Got the latest disk image (0.5.1) and opened it up. In there were two folders, I took the Planeshift folder and put it into the Application folder in my home directory (drag and drop worked fine from the .dmg to the finder folder). Now, putting it in that particular folder is probably questionable, but apparently it can be dropped pretty much anywhere.
After it is copied, open that folder and double click the pslaunch application. I told it to update which it did (I assume to update the launcher), and then I had to do it a second time (which took longer, so I assume it was updated the games files). Then, I selected the settings to get the appropriate screen setting for my machine and saved them. Then hit run and the launcher subsequently died with a dialog box saying it encountered an unexpected error and went away. I ran the other executable in that folder (the name escapes me but it begins with ps and isn't the launcher) and the game started up.
Just out of curiosity, checking in my home Library/Application folder and there is a Planeshift folder that contains what I think are the settings.
So, I believe it to be installed, I don't know why the launcher doesn't run the game, but you can just run the game itself and it appears to run.
I haven't gotten past the tutorial zone so I can't speak to playability, speed, etc.
