I love it. I just wish there were more upgrades.
The initial haul to level 3 in all the basic upgrades seems to take forever, but after that you gain thousands upon thousands of utterly useless RPs. I wish there were more upgrade levels for those who are really, really dedicated to playing it through and seeing all the game modes and such.
It would also be nice if the highway actually had visible on/off ramps, even if they were totally barricaded off with concrete road barriers, chain-link fences and piles of other stuff, especially if there were zombies pounding on the outsides of them.
As well, killing the choppers is an aggrevating pain in the ass, as mentioned, because your options are to hammer into wreckage and die in order to steer the chopper into signs, or play it safe with the wreckage and get your car hammered into wreckage by the chopper's guns and missiles. This might be partially alleviated if you made signs poles across the whole roadway with tall signs in more places than just the extreme left and right, or at least added coding to ensure there are no cars directly in front of the next sign.