The D5100 has served me well and it still remains a pretty highly rated, and in my opinion, remains the best entry-level DSLR because of its perfect combination of price, picture quality, and features. I bought it a couple of years ago and have since mastered its features, and I'm ready to move on to a full-frame.
The moment I began shooting with the D800 , I realized a couple of things very quickly. Its very bulky yet very well-built when compared to the D5100. Secondly, the controls were NOTHING like the D5100, which means that I'll need to re-learn how to shoot quickly. From what I've read thought, it seems that at the pro level, Nikon seems to keep the controls pretty consistent across their cameras and it's something that they kept in mind when they designed the D800. It was mad confusing at first, but some Youtube and my buddy Fred Assad helped me find my way.
The look and feel of the camera are AMAZING, and this is extremely exciting for me to actually see and handle a pro level camera. Everything from the sound of the shutter to the size (I've got big hands) are amazing.
The camera has one SD slot and one CF slot. I have 64GB SD and a 32GB CF card that I keep in the camera. I've read that RAWs at full resolution (36.2 MP) are a minimum of 15 mp each, so I also bought a couple of internal card readers for my work and home desktop, to make transferring these files faster and easier.