Windows 7 SP1 brings two big features– Dynamic Memory support and a more robust Remote Desktop client which uses RemoteFX. and the other part is the large number of Bug fixes.
It’s been leaked and is freely available to anyone willing to download and install the update from a torrent site.
Though the update is legit, its hard to say what else is included in the update.
After the update, the main build number has been incremented by one, so you now have Build 7601. (This is exactly like Vista SP1)
The system was stable for the period I tested after the update. however, you can try it at your own risk.