I'm slacking, and just finished up the IX event last night. Jecht BSB is one of the first BSBs I got, and I've never been horribly impressed with it nor used it much. It's 8 random dark attacks and attach dark, with super basic 4 hit dark single target and 2 hit dark AE commands, with no bonuses at all. Especially since dark vuln is so rare, it's hardly been useful.
Well, here comes Beatrix and Steiner, both vuln to dark. After failing CM a few times with minimal synergy, I just went with a regular mixed team and threw Jecht in there. His first BSB, unimpressive. They probably weren't fully def broken, and the attach dark obviously wasn't up yet - 4k x8 split between the two randomly. Got a few more breaks up, plus the attach, and his command 1 is dealing 4x 8k to beatrix. Nice. Wiped her out fast. By now Steiner is fully broken, Jecht is fully buffed, and his BSB came up again, with the attach dark still up. 8x 9999 to Steiner. Blew his ass UP. I haven't pulled any OSB, so this was oh so satisfying.
Then destroyed Zidane because he's a pussy.