Yeah, they are worth it. And they do get progressively harder. CCNA is pretty simple. CCNP require a lot more work and studying. A ton more material, not to mention 4 tests versus 1 for CCNA. Once your to CCNP level and you have a year or two under your belt of working with the stuff, there's no reason you should't be making 75+. And once your a CCNP and have some experience I would definitely pursue CCIE. The written test is difficult, but most can pass with enough studying. The 2 day lab is the real bitch. Most fail one, two, three or more times before passing 🙂 Can become costly at $1200 a pop, but if you work for a company that knows your going to be a CCIE they'll often put up all kinds of money to help you in your quest, because CCIE's are still very rare and Cisco employs most of them.
I'm currently working on my CCNP.