I have a Ted Cash oval-shaped capper. Works pretty well, but I tend to prefer fishing out caps, one at a time, from the original container, and sticking them manually onto the nipples.
Several reasons. Even with the capper, you need to fish out the caps and load them, one by one, into the capper. Then, at the end of the day, if there are caps left in the capper, I need to unload them and put them back into the original container, again with my fingers, risking dropping.
And when I put on a cap with my fingers, I can immediately seat it onto the nipple with my thumb. With a capper, I still need to seat the cap, with an extra motion.
Cappers aren't foolproof - the caps can hang up, jam, etc.
The chances of dropping a cap on the ground using a capper are still very possible.
I shoot only at the range, where speed of reloading isn't critical. Maybe a hunter might get a second shot using a capper, but I sort of doubt it.
Just my 2 cents.