Damn....Oli Geddes has been pulling halfguard forever. Because he filmed such huge number of his matches, his grappling career on youtube is actually a great database of halfguard in competition.
Here is a podcast where he makes some interesting observations about the halfguard for a guy who uses it to the great extent he does. He says:
"the halfguard is not mechanically strong enough to not suffer while using it."
"if you stay there for too long...it will not favor you in the long run."
"there aren't that many submissions from there...so you have to sweep, but if you sweep you have to then pass, so if you're going to do that...why not just do takedowns and pass instead of suffering in halfguard."
[All paraphrased]
Obviously, there are a lot of matches with 10thP guys (including Eddie/Royler II) that are good counter-points to his comments. But still, it is really interesting to hear such a big halfguard guy say these things.
http://www.openmatradio.com/?p=1731