So far, I tried out Thousand-Year Door, Pokemon Legends Arceus, Breath of the Wild (not upgraded), Pokemon Shield, Super Mario Bros. Wonder, Wavetale, Cuphead, and Smash Ultimate. Of these, Wonder and BOTW got the most noticeable improvements for me, though the rest of these games also are now looking much better (except Cuphead, which never looked very crunchy to begin with and looks indistinguishable from pre-Handheld Boost).
This update will make me want to revisit a bunch of my Switch games.