GreenAddress is safe to use. It uses a central server but your private key is never shared with server and also even if the server is down, you still have the chance to get it using 'nLock time'.
GreenAddress will send a transaction with nLockTime whenever someone makes a deposit. This transaction sends the funds back to the depositor. The nLockTime transaction effectively acts as a deposit expiry: after this time, it will be included in a block and the depositer will get the funds back.
See
https://en.bitcoin.it/wiki/NLockTime.
I've never used it, to be honest. I think even Mt. Gox supported in back in the days. I wonder why they went out of the way and implemented support for it. What do you think is the reason it didn't catch on more?
Please don't spread FUD! Make a good research 'what it was'!
Is it is confirmed that Mt.Gox is supporting the ga. Do you have proof. If they are supporting it then i dont think this is a safa to store btc
He is going away from the truth. What it was is: Mt.Gox's GreenAddress is different from this. They created their own not they supported this wallet. This wallet was released after Mt. Gox was shutdown/hacked and how could they support this even before the release?
It is safe to use GreenAddress as it provode signed time locked TX which you can use to get your coins after the lock is expired. You can do 1 day trial or you can also use their testnet wallet before using mainnet if you like.
Edit: 'Green address' like style was first implemented in Instawallet. See the ANN:
https://bitcointalk.org/index.php?topic=32818.0 and then in Mt. Gox, ANN:
https://bitcointalk.org/index.php?topic=48170.0.
-MZ