ShadowPlay is cool, if you use the Shadow option and have a smaller SSD as a boot drive, make sure you change your Temp path - it records your buffer there by default and then dumps it to your record path.
Only downside to Shadowplay is it doesn't record your local mic input. You could loop yourself back into the audio stream and mute the output (should still record on the audio track) but the fact it doesn't do that is pretty lame. FPS impact on my 770 was negligible, 5-10 FPS tops. Even OBS sucks down some CPU during encoding on the fly, nothing out there can really touch Shadowplay. I'd like to see some more encoding options (it's all h264, options are low, medium and high - that's it). Defaults to 1080p 60fps as well, which is fine, but it'd be great to have other options.
All in all, it's a cool feature, but needs some refinement before it's ready for primetime.