I've ridden SFNE's Flashback a total of once, and I enjoyed my experience. It wasn't rough, and the drops were pretty good. The inversions were even pretty decent. I could easily see myself getting sick of them after multiple rides, though.
As for SLC's, I rode SFNE's Mind Eraser yesterday for the fifth time and got off the ride thinking, "Man, if this coaster weren't so rough, I'd love it. The layout is actually very interesting with a lot of good foot choppers. I also love the 'drop' after the heart-shaped inversions. I think the turn-off for most enthusiasts about these coasters isn't the ride itself, but the roughness, and the fact that so many parks (primarily Six Flags parks) add these lower quality rides instead of B&M inverteds.
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"If we built a ride that everyone wanted to ride, it would be called an elevator, and that's not an amusement ride."
-Stan Checketts, S&S Power