Earlier quoted context omitted.
Actually no. Why is it so popular? It seems to have fewer features than hipmunk.... Not to knock them, though, it's a hard problem to solve well from an interface and integrations perspective.
Personally, I reach for Booking.com because they're simply better than most competitors. They have a huge inventory of hotels, and they have a functional, fairly responsive search interface that isn't annoying. The last part is important — there's little in terms of popups, ads, intrusive upsells, flashy UI elements, and so on. It's not the most modern, but it's very well optimized towards finding stuff. For example,…
On the other hand, do note that the Booking.com review scale is skewed - it's seems to actually be a 2.5-10 scale. The median rating is something like 8.1, so keep that in mind if you're using the review scores.