I'm probably lower-intermediate at Japanese, so take this with a grain of salt.
I find that reading vertical text feels better, even though I first learned to read horizontal text. I don't know if this is all in my mind, or it really does have some appeal to it, though.
Ah, yeah, my main motivation is reading light novels, and I've actually read a few really easy ones. I've also read manga, but it's hard to find any that I like that are also at my level.
I'm not sure how I'd feel if the vast majority of my usage had been horizontal instead.
I find that reading vertical text feels better, even though I first learned to read horizontal text. I don't know if this is all in my mind, or it really does have some appeal to it, though.