I'm running libva 1.6.1 and libva-intel-driver 1.6.1 on Sandy Bridge.
If I encode any video with low (< 8 Mbit/s) bitrate I get very distorted video output with horizontal stripes in it. This is a case for both cqp and cbr encoding.
I've tried to encode it with gstreamer 1.0, avcenc and h264encode with the same output result.

This is what I get with 3000 Kbit/s CBR encoding:
http://ovrload.ru/f/55938_bake-cbr-3000.mkv

It was encoded with the following gstreamer pipeline:

gst-launch-1.0 -ve filesrc location=bake_op.mkv ! matroskademux ! \
      avdec_h264 ! videoconvert ! video/x-raw,format=NV12 ! \
      vaapiencode_h264 rate-control=cbr bitrate=3000 ! \
      h264parse ! matroskamux ! progressreport ! \
      filesink location=bake-cbr-3000.mkv

(source is ftp://serv.valdikss.org.ru/Anime/Bakemonogatari/Bonus/%5BANE%5D%20Bakemonogatari%20-%20Ep03%20Creditless%20Opening%20%5BBDRip%201080p%20x264%20FLAC%5D.mkv)

Windows encoders like MediaCoder and handbrake produce decent video quality without horizontal stripes. Nothing different with current git master of libva and libva-intel-driver.

See also: https://bugzilla.gnome.org/show_bug.cgi?id=751885