Booking.com is huge, they're part of the same group that owns Kayak, Priceline, etc. Now, I have friends that have had really bad experiences with Priceline.
I think the general rule of thumb if you're worried about having a bad time is to just stick to the big hotel brands and book directly.
Personally I use hotels.com and never had an issue but I only stick with the big chains (Hilton mainly).
Caveat: when booking directly you still need to confirm your reservation by phone.
I have twice had Wyndham hotels book for earlier dates than what I had input on their website when making the reservation. (After it happened the first time, I triple-checked the input fields, and it happened again.)
It appears that this hotel chain has a glitchy website. Traveller beware.
I think the general rule of thumb if you're worried about having a bad time is to just stick to the big hotel brands and book directly.
Personally I use hotels.com and never had an issue but I only stick with the big chains (Hilton mainly).