Usability on booking.com is decent, search, filters and map view are useful. Things have been getting better elsewhere, so it probably doesn't stand out as much as it did years ago.
Either way, if prices are roughly the same on most sites, I'll go for something that just lets me get the job done. I couldn't care less about aesthetics and commissions if other places can't give me a much better discount while also not getting me angry using their site.
booking.com is such a piece of shit in so many ways: strip-mall aesthetics, horrible data quality (firsthand knowledge here), huge commissions for hotels, and yet, it's a fantastic business.
Probably a good lesson or two here.