Accidental discovery, but have to say they've helped far more than hindered (especially with last minute bookings in distant lands).
Booking blames this on the hotels (probably true to some extend), but the problem is quite widespread so that Booking probably bears some responsibility, too.
I always book direct with the hotel. Saving $5 on a room is not worth showing up at 11pm to find out they don't have your reservation.
On rare occasions, using a 3rd party site will net you a better deal, but often, it's the exact same price, or a price within just a few dollars.
But if your reservation has a snag of any kind, you're basically fucked. The hotel will tell you to talk to Booking, Booking will tell you to talk to the hotel, if you can even talk to anybody.
I'd rather spent the extra dollars and book directly and just not have that worry.