Firstly, if I really love the formula because if I personally LOVE a product then I am more inclined to keep it regardless of its usefulness to me as a reviewer. If it’s exceptionally bad across the board, then I’m not holding it at all because who else has to hold on to compare later?
Otherwise, I look at the brand to see if I think it’s popular enough to save for possible comparison or to be able to refresh my memory for formulas later. If it’s a limited edition product, I’m less likely to keep it these days. I don’t have space and I don’t go back to the archives as often as I did years ago (because my photo quality is more consistent and has improved over time).
Lip products are really about a formula that I really want to wear again. I know I very, very rarely reach for a liquid, matte, or even all-over matte lipstick, so I find myself passing these on sooner rather than later. I like a lot of colors, and since I am saving the price, I’ll keep entire formulas over just a few shades if I really love the formula. It’s very rare that I keep a shade and not its sibling!