I've never used them much at all, other than very rarely if I happen to remember to use an X Accuracy for something like a Sand Attack spamming Pidgey. I generally sold them back when there was a limit on bag items, but now I don't bother.
The concept seems pretty good - the benefits of a stat-boosting move without having to use up a move slot - but they work out to just not be that useful. In-game, stat-boosting isn't particularly necessary, since you don't need to sweep, since the AI is dumb as a brick and switching is pretty much risk free. And competitively, when stat-boosting becomes a more desirable strategy, you can't use items anyway.