I don't know what ground the demo covers, but it felt pretty much the same all the way through. There are some jet-pack sections, but they are (sadly) brief. So if you liked what you saw, it's a few more hours of that.
As far as difficulty: it does get harder, naturally, but it's manageable. Save your special attack (forget what it's called, the hulk out, dish more damage while invulnerable thing) for the big enemies, take note of which weapons are best against what type of enemy, don't stand out in the open unless you are actively killing things, pick your targets, etc. I played it in one glorious sitting and there was only one particularly frustrating section, right at the end, but I had overlooked a really powerful weapon in the environment and with that it went from frustrating to very, very satisfying. I had some deaths along the way, but it never felt cheap.