I have the Metz Mecablitz-15 ringflash and I quite like it and it's a well known name in flash equipment. It supports TTL for a number of camera brands, including Canon, Nikon, and Pentax and comes with a 52, 55, and 58mm lens adapter. One of the things I liked about, and I shoot Pentax, is the cross brand support which means it's still good to go if I ever switch (which, admittedly, is unlikely).
In terms of some feature notes, by the way...
- The two sides can be independently controlled.
- The flash is upgradeable via USB in the event that wireless protocols change.
- Amazon is listing below $350. Okay, not specifically a feature, but a very good price for a very good device.