FWIW, the O-GPS goes for about $200, not $350. You can find them on sale at times for even less.
o-gps1 in Camera and Photo Equipment and Accessories | eBay
The biggest two quirks (for lack of the better term) are learning where and where it won't work and how to properly calibrate it. The instructions are spotty at best in explaining both.
Since the unit uses magnetism to track, if you set up somewhere with a lot of metal in the ground (example, by the river here it used to be a mill, the ground is filled with iron slag and the tracker is almost useless there) it will not behave itself.
It will work perfectly out my apartment window, but will be thrown down on the ground level here for the reasons I mentioned above.
As for calibration, that just takes a bit of practice. You'll get to 'know' when you have and when you don't after a while. Once properly set, you can go all night without needing to retool it.
EDIT:
As another example, heres the Orion Nebula @ 300mm using a funky old 80's manual 300mm lens. This was either f/5.6 or f/8, 60 second exposure on the same borked K-30.