Thanks for replies.
If I got it correctly an attacker can secretly mine a chain of blocks. If he reveals the chain, and his chain is longer, he will then reverse all transaction back to the first block of his chain. If he has more than 50% he can routinely do this, hence make Bitcoin practically unusable. An attacker with less than 50% can only do this at occasions and cause minor problems. Correct?
Correct, and an attacker with less than 50% does this at great expense to themselves. Every time that they don't immediately broadcast the block that they've just solved, and instead keep it hidden while trying to keep ahead of the rest of the network, they risk the block reward of that block and the reward of the rest of the chain that they build while they try to catch up if they fail to surpass the honest network.
If they are trying to reverse 6 blocks, and they only manage to stay ahead of the honest chain for 5 blocks, Then they lose out on at least 125 BTC today (plus any transaction fees they could have claimed if they had included fee paying transactions in those 5 blocks).