You weren't the only one to buy GripShift- I have it as well. Just didn't bother with the online play at all. I've deleted it for now- if i want it again it's in my download history.
When you do end up ending the game any planets you have unlocked stay unlocked. When you fire up the game again you can choose to go to any planet you have available. It saves just at the end of the game- no pausing to save required.
When continuing during the one session (say, game over in the second planet and decide to start there for another session right away) your weaponry will stay powered up. If you leave the game and come back to it (checking the messages, folding, etc.) you'll have your weapons at basic strength and are better off going back to the start of the first planet to power up again.
Replayability is excellent- i easily find myself killing hours trying to get just a bit further in. It's more of a quick "pick up and play" arcade title in the same vein as Asteroids or other outer space shooters.