Receiver-mounted bike rack

I know next to nothing about bike racks. I am interested in getting one that mounts in a receiver, for two bikes. Don't ask what kind of bikes, cause we don't have them yet. :) But they will be fairly generic bikes, not expensive ones.

I want the carrier to hold the bikes securely, and I'll need some way to lock the bikes into it. I'm not looking for the cheapest way out. I want something that works decently.

What to look at, and what to avoid?