I liked it, a lot!
I finished the game and almost all got the upgrades maxed (except damage). I tried the continue one other round at the last but when i blasted all the enemies they were nothing left... only power ups passing by...
Anyways, the key to this game is to use the indirect fire like a mortar or to hit more than one enemy at the same time. Direct fire can still be good but the best is to have a wall of marbles coming from the sky. You will have to upgrade your marble ammo for this. I used monster balls and even if the price is high this is a good addition to your arsenal.
As a side note, i found that the marble shooter looks like a distant cousin of Wall-E... but maybe it is just my imagination! :P