I have a pretty good success rate over the last few years doing miles + copay as a GLD. I've done it multiple times to/from LHR, and twice to Sao Paulo. They don't always clear, but I'd estimate about a 60% success rate. Not too bad.

With the changes to qualification in 2016, I am looking to spend more time in paid F (domestic) or J/Y+ (international) and do less upgrades, so we'll see how that works out.