I think the checkpoints are about at least a month or two back in time. Anyone who has more than 51% of 6.5 Petahash of mining power that can fork their own chain past the latest check point, is better off mining regularly and getting 51% of 3600 coins daily or about 1800 coins, at least for the next 2000 blocks when the difficulty will double.
Of course hidden fork crunched at high speed in shadow is unlikely, and maybe checkpoints do not harm due to low probability of such fork.
But they harm in other way - they compromise Bitcoin decentralization, which is in my opinion is the biggest attractive feature of Bitcoin.
Today they add checkpoints (this effectively changes protocol specification), tomorrow they could freeze BTC on your address, by adding small tweaks to validation routine (just like checkpoints validation).