I don't really know how long this GUI called Hybridhas been around, but after finding it today, I thought I'd do a few test encodes.
The first was a conversion from dvd-video to mkv. Hybrid uses the X264 encoder, like virtually all of the free H264 encoding programs, so quality of output should be good to excellent, depending on your choices in the options provided. I used the slow encoding option, and a single pass CRF at 20. Output was High email@example.com, and I let the AC3 audio pass through unchanged.
Going from a 4.3gb dvd, the total size of the new mkv is 1.12gb (running time of the movie is 89 minutes). I forgot to time how long this first one took to complete, but it didn't seem excessive.
Playback is fine, no obvious glitches or audio/video sync problems. Picture quality is, as expected, excellent.
So what are the downsides? Well, the interface of this program will send newcomers screaming into the night. Its really not that hard to manage, it just looks ultra geeky, and gives me uncomfortable flashbacks to MeGUI. Though even MeGUI seems more polished.
If you want to work with AAC audio, you'll need to download the Nero AAC encoder and put it in the Hybrid folder, but that is explained when setting up the program.