Headers first changes the way in which the Bitcoin Core client discovers a chain, downloads and verifies it. Pieter Wuille, who has been working on the change, is calling for people to review and test his code.

The patch, which is available via a github pull request or packaged here (includes binaries) introduces the following changes:

• Parallel block downloading (much faster sync on typical network connections).
• No more stalled downloads.
• Much more robust against unresponsive or slow peers.
• Removes a class of DoS attacks related to peers feeding you low-difficulty valid large blocks on a side branch.
• Reduces the need for checkpoints in the code.
• No orphan blocks stored in memory anymore (reducing memory usage during sync).
• A major step step towards an SPV mode using the reference codebase.

At the protocol level, this patch increases the minimally supported version to bitcoin v3.18 which was released in December of 2010.

At this time, there are a few known issues which are further described on the bitcoin mailing list found here.

Source – http://sourceforge.net/p/bitcoin/mailman/message/32921390/