I've used mine a couple times, but not due to "round irregularities" or whatnot--it was inevitably mag-related issues when it was just
barely hung up/not "fed" enough to finish the path into the chamber. The couple times were both with the same mag (which is a "range mag", so I don't really worry). Once I also needed to use it when a buddy had packed up already at the range, and forgot about one or two partially-loaded mags that he offered me to shoot, and it was also mag-related (but once again, it was a "range mag" of his, as well).
Ideally, you won't *need* it, especially if all of your equipment is well maintained; however, there wouldn't be a FA at all if there weren't times where not everything was perfect, equipment-wise.
Granted, I'm right-handed, so it doesn't bother me that it's there, and granted the weird "mag issues" related above
could've been solved with my thumb rather than the FA....but I didn't feel like feeling the bolt with my thumb by that point after having been shooting