The unfortunate truth is that, right now, hardware-accelerated video transcoding on the PC is a mess.
Support for black-box encoders is spotty. We saw output quality at the same settings vary wildly depending on the conversion software used. Not only that, but none of the black-box encoders we used matched the quality level of unaccelerated software conversion. Sometimes, the differences were glaring, with the black boxes producing a ton more artifacts and adding ugly jaggies around hard object edges. The only upside, really, is the encoding speed. For some folks, maybe that's all that matters. Maybe it's simply about getting a big video down to a manageable file size in as little time as possible. If you're going to be watching the output on a 4" smartphone, perhaps that isn't a bad approach. Artifacts may not be visible or noticeable on that small a display, making encoder quality very much a secondary concern.
It's a shame, though. Four long years have passed since Elemental released Badaboom 1.0, and we're still facing a heavily fragmented ecosystem with vast inconsistencies in performance and image quality.
There may be hope on the OpenCL front. As we've seen, the OpenCL-accelerated version of x264 can produce relatively consistent output on different hardware. However, only a portion of the encoding pipeline is accelerated, with much of the work still being done on the CPU. On our test rig, substituting the Radeon HD 7750 for a much quicker Radeon HD 7850 didn't substantially reduce encoding times—they were still just over 30 seconds. It's possible some optimization work remains to be done. After all, we were using a beta, and the x264 developers haven't released a public version of their OpenCL-accelerated software yet. Still, we're not completely sold on the effectiveness of OpenCL acceleration here.
For the time being, the best option for quick, high-quality video transcoding is unfortunately to buckle down, get yourself a fast CPU, and run the best software encoder you can find (which may be Handbrake).
If performance matters to you more than quality, then using QuickSync in MediaConverter might be a suitable option. Encoding times will be very short, and image quality, while poorer than with Handbrake, will be adequate, especially if you'll be viewing the video on a smaller screen. Other hardware transcoders were slower than our CPU in MediaConverter, though, and we were generally unimpressed with the image quality of the hardware solutions in MediaEspresso.