When choosing mounts, I've tried to get shallow mounts (as close to the wall as possible). These tend to be the simple ones. The complicated ones that pivot, swivel, rotate, etc., all hang out farther away from the wall to accommodate those mechanisms. The more the TVs hangs out away from the wall, the less "pretty" the installation is. Also the TV will wobble more, even from simple things like someone walking past nearby the TV.
Typically you'd hide cables by knocking a hole in the drywall and dropping them down inside the wall to the floor where you have another hole in the drywall for them to come back out. If you want to be fancy they make plates with all the kinds of connectors you'd use for A/V. However, I assume you cannot do that with a fireplace. Alternatively you can buy those conduit-like cable channels and run that down one side of the fireplace. Another option is to go wireless. In that case you just need an electrician to add an outlet behind where the TV will go so that it gets power. Then there is a separate connection box you can hide away that you make all your A/V connections to, like cable boxes, DVRs, AppleTV, whatever. That connection box sends the HDMI signal wirelessly to the TV.
This should go without saying, but the mounts must be screwed into studs or brick or whatever is solid behind that drywall. I saw a picture of someone's TV that had fallen because he had only used drywall screws. He used those huge screws that claim they support 50 pounds each, and figured 6 of them would be rated for 300 pounds total, which was many times the weight of his TV. Well that didn't work out for him.
Personally I don't like looking up at TVs, so I would only mount it there as a last resort to anywhere else, and if I did mount it there I would mount it as low as possible.
