I think a 51% attack would be a disaster. But someone with 51% of the hashing power does not necessarily mean they are attacking.
The chances of profiting from a 51% attack are just ridiculously low. Most exchanges need you to wait 3 or 6 confirmations. So basically the one attacking has to find the block 6 times in a row to be able to doublespend properly. Even if that would happen most exchanges would notice that the payment is a double spend. Chances of finding the block 6 times in a row aren't that high, even with 50% net work hashrate.
Basically you have a 50% chance to find the block.. 6 times in a row.
With lower hashrate you still have a chance to do it, but I would rather you'd pick up gambling instead. Your chances might be higher to win there.