I'd mentioned the Great Wall Fuseki in the past, but recently I'd started up a slew of games on OGS and started most all of them with this opening, including some as white.
I do believe that this fuseki is inferior to traditional openings, if only because professionals do not use it. Also because I tend to consistently do worse with this fuseki than if I'd tried a normal opening.
But, I will say, it is fun, and definitely a thing to try if you're playing even against a weaker opponent. Since my massive rank drop on OGS due to all my games timing out, I'd been playing lots of mis-handicapped ranked games against much weaker opponents (like 20kyu, I estimate I'm still about 6kyu on OGS). I kind of feel bad about it, but what can I do? That's what my rank is.
Oh well, back to the Great Wall (of Death).
The breakthrough I recently had, besides lots of practice with it, was rereading Bruce Wilcox's material on it from Go Dojo: Sector Fights. Although you must remain flexible with this opening, one thing to aim for after the wall is built is to play inside (on the board sides opposing your wall) and try to create a massive moyo on these sides.
The problem I always had was that you are almost guaranteed that the opponent will not let you do this by invading on that side. The breakthrough for me was realizing that, according to Bruce, this is your goal. You want the opponent to invade, and by make such a huge moyo this is almost guaranteed.
His idea was that this type of playing allows you to control the fight by constantly attacking the invaders and utilizing you running and attacking to attack the corners. You're not likely to kill the corners, but you are likely to reduce them and split them from each other.
This leaves the opponent with many separated groups, and you with one giant group and possibly some other groups that can be easily joined with it. You may lose the big moyo, you may not, but you retain control of the game most of the time, and with steady aggression you can often turn this into a win. In Bruce's Sector Fights thinking, your stones all work together, the opponents are split and do not work together. This theoretically makes each of your stones work harder.
It's a scary opening to play, since you start so far behind in terms of potential territory. It's been working out fairly well for me (against much weaker players, granted) and it's definitely fun to play.
People often try to interfere with the building of the wall, but that's fine, that's where the flexibility comes in.
Maybe I'll discuss it more later if I ever get time (unlikely), but the Go Dojo material discusses it better than I can anyway.
(Oh, the "of Death" part I added because you end up playing a very aggressive game where you are constantly chasing and trying to kill the other groups. If you aren't killing or threatening to kill enemy groups after playing this fuseki, you will probably lose).
Here's an in-progress game (just waiting for pass/resign). It shows a situation where one of the giant moyos actually got completed (bottom). Of course, this was against a 21kyu I think, so... yeah.
I could not create the side moyo at the top, but I was able to take the corners instead (flexibility). It's all one big black group except for the upper right corner. Was able to reduce lower left corner fairly well, even without white invasion. Click image for SGF.