As far as I can tell, for most users a 51% would at worst look like their transaction didn't "go through". This is because wallet software waits for the system to mine a certain number of blocks before showing the final wallet balance. A 51% attack that is 1 block deep may actually be a good thing for bitcoin because it would cement the idea of needing to wait for confirmations in the minds of users. In this way, 51% attacks do not kill bitcoin, but instead the attack simply makes the network appear as though it is moving more slowly than it used to because users would need to wait for more confirmations.