As of v1.3.0 build 25, Switches supports the 1.6.4 client.
As of v1.2.0 build 21, Switches no longer requires CCC for special piston features (flipping levers and clicking switches).
It still goes into the /mods folder (as there is no /coremods folder in 1.6.2).
As of v1.1.0 build 19, emerald and diamond switch designs were added.
In the earlier versions, only iron and gold switches are available.
How It Works
Switches are placed and respond just like vanilla levers (you can place them on any side of a block).
Right-clicking a switch toggles its state between on and off.
You can toggle a switch (or lever) by having a piston extend directly against it. A retracting sticky piston will also pull a lever back towards itself.
Switches made of different materials do not act differently - it's purely aesthetics!
See the FTB wiki page.