I have the flash I and II and they are ok. I actually hate the Flash I as it's handle is a scale module of the Flash II. The scaled-down model doesn't take into account that once it's smaller it wouldn't fit a normal person's hand/fingers anymore! The Flash I is probably one of the least comfortable knives I own (and I own about 20 folders).
At first I didn't like the Twitch XL but it's really growing on me. I've been meaning to do a review for a while now. It's my favorite IWB knife. It's thin, sturdy and I figure that the 'flipper' will be easy to deploy when under stress or distraction if needed in an SD capacity.
The Twitch's lockback is kinda annoying, as is the stitch to turn it off. The lock is solid, though. I just superglued the switch into the "off" position because I kept putting it on accidentally and then be unable to open the knife quickly.
I waited about 3 weeks before I glued it, wondering if it was a good idea to do that. I figure that the SuperGlue isn't strong enough to really hold it. I think I can break it open anytime (that's my plan, anyway). It's never opened accidentally, I don't see it happening easily.
The Flash's lock works well, but feels uninspiring. The clip on the Flashes are great! Low-riding, unobtrusive, but is easy to grab and extract from the pocket because of the way it's designed.
The Twitch XL's clip would be less obtrusive if it didn't say SOG on it. It looks cool but calls way too much attention to itself.
Overall, I like the Twitch better than the Flash. That's primarily because of the flipper, but also because the construction just feels much sturdier.