I've done about a dozen FDT's trips mostly on Delta metal. Here's what I've found.
Aircraft Jumpseat Info
1.) CRJ2 - Tight fit for shoulders and legs, Seat slides out from flight wall in front of the flight deck door. Cramped ride all around, try and avoid long distances. Standard rollaboard carry on will have to go in the closet outside of the flight deck.
2.) B739 - Just like a CRJ2 but with a comfortable amount of leg room (seat still slides out of wall at the flight deck door). Cramped but tolerable for longer distances since you can move your legs slightly. Standard rollaboard carry on will fit in a compartment behind the captain.
3.) A319 - Two jump seats, one on each side of the flight deck. Both fold down from the back flight deck wall. A comfortable amount of room, plenty of shoulder room and a just ok amount of leg room. There's enough room for both jump seats to be occupied and be able to shift the way you sit left/right for long flights. Standard rollaboard carry on will fit under your flap of a seat.
4.) B752 - One jump seat behind the captain that folds down from the rear flight deck wall. When the captain pushes his seat back (and most I've rode with do) you have 2-3" between the back of the captain's seat and the end of your flap to sit on meaning you will need to sit sideways for the entire flight. I did a ATL-SLC ride this way and it started to become quite painful after the first 2-hrs to be constantly contorted sideways. I would try and avoid this ride for longer flights if you have to sit sideways. But otherwise it's a decent view as it's theater style seating sitting about a foot above the pilots with a window. Standard rollaboard carry on will fit under the flap of your seat.
5.) B764 - One dedicated jump seat behind the first officer. There is a ton of space here and the seat is comfortable since this is an actual seat and not just a flap that folds out of the wall or from behind the wall. You may actually feel far away from the pilots when they turn around to talk to you during flight with how spacious the flight deck is. Ideal for longer trips. Standard rollaboard carry on will fit laying down in any nook or cranny.
6.) B744 - There are three dedicated jump seats; two behind the captain and one behind the first officer. These are actual seats and not flaps that fold down from walls. The seat furthest back is less than ideal since you will barely be able to hear the pilots during flight unless they shout with how far back you are. You also see nothing but sky during takeoff and landing from back there. The seat behind the first officer is the one you want because it will slide to the center of the flight deck. Ideal flight deck for long trips even with all three seats occupied. Mine was unfortunately a short ATL-DTW hop. Standard rollaboard carry on will fit laying down just about anywhere.
Tips/Tricks
1.) People at the airport understand "jump seat". People don't understand "flight deck training". The sooner you switch terms, the smoother things will go.
2.) TSA has no idea who you are or what you are doing. In order to avoid waiting for a supervisor I've found it is simpler to go to the ticket counter first and tell the agent you are trying to jump seat on a flight. Same deal as someone mentioned above, when they ask you for your airline just tell them to use ATC and they'll find you in the CASS. Once they do, they'll print out a boarding pass for you and this is what will let you breeze through TSA. They won't even give you a second look once you hand over a boarding pass from the airlines instead of the handwritten one your ATM or supe will give you.
3.) Even though you get a boarding pass from the ticket agent, you still need to go to the gate and have the gate agent check you in to the jump seat manually. You won't be able to board until they check you in manually at the gate.
4.) You are technically last on the priority list. However it pays to show up early and be first.
5.) If you are trying to FDT on one of your RDO's, get your RDO's swapped around before you submit your FDT paperwork.
6.) If you have lounge access via credit cards or other avenue that gives you access while flying on that certain airline that day, the jump seat boarding pass will suffice for entrance into the lounge. With the handwritten ones from your ATM, your mileage may vary.