ok so when you enter the code it allows you to call up the number in-game to have the blimp dropped.
I did this as Franklin and there was a short dialouge of him ordering the blimp. After that a blimp icon appeared on the map. I had to drive down to the airport so I could get in the blimp. It is indeed slow. It's kinda neat, and reminds me of the UFO pre-order vehicle in saints row 2 (though IMHO that one was more fun)
I have the Atomic blimp (code insert in the game box) and the blimp is very good. It flies decently fast (maybe as fast as the slowest chopper in the game and way slower than planes) so you can traverse the map quite fast with that blimp actually. Even though the map is decently big it is nowhere near as big for the fast planes. You will run out of space quite soon with the fastest jets and have to turn around. Just Cause 2 map is way way bigger here given how much I love flying the jet in that. Still, GTA V map is big. Just don't expect it to be super huge. I think blimps are a good mode of transport
Landing it is tough unless you have space. It could blow up if you're not careful. The handling is great but the one drawback is how the camera keeps snapping back to primary view when you move the stick around.
Looking down at the roads and terrain from the top is a simply wonderful feeling given you can keep the blimp hovering at a spot. Also, don't invade air-space of commercial planes or other airports or the Fort Zancudo military base unless you want to be shot down fast.
Overall, I'd say the blimp is a great addition and I love flying around in it. If you lose it, you can call one again with a phone call and it appears at the closest drop off point near you.
For me, I don't think the Blimp is that great. I was hoping it was going to be like a cheat and the blimp would spawn wherever you are. But you have to call them up and go to the airport to pick it up. It's cool to have access to a a blimp right away so you can do some air travel, but I don't think it was great for the pre-order incentive.
I just like to jump out of it. With or without a parachute.