Far and away the most difficult thing was the paperwork. Studying mechanics of materials is a fairly straight-forward affair. There are dozens of texts dedicated to the subject but there are zero books on the subject of paperwork related to the PE exam (or at least none of which I am aware).
Not all states have the same requirements for registration as a PE. Some states require character references and interviews similar to those required of applicants to the Bar. Illinois requires documentation of at least four years of engineering experience with an employment verification form that is to be filled out by your supervisor for every job that you use list as an experience credit. You also need transcripts from you college and some deep pockets to cover the cost of all the application and examination fees.
The tricky part is that in Illinois all of the testing is carried out by third-party company. In order to sit for the exam you must send all of your forms to this company and they will determine if you are eligible to sit the exam. They require all paperwork to be completed about two months prior to the exam but they do not send you any indication that you have been accepted until about ten days before exam day. There must be people out there they have diligently prepared for the exam only to be turned away ten days before showtime. After sending your paperwork to the testing company, you also will have to send everything to the State, as well. If you do not, your application will be regarded as incomplete and you will not be granted a license until you complete all of the required paperwork with the State.
I missed this little fact and my license was delayed a month or so. This was on top of the two to three month wait for the exams to be graded. All things considered it is a very slow process, but as it ultimately could lead to people living or dying, it is understandable that the exams must be graded very carefully.
Oh, and in regards to the need for deep pockets, sitting the FE and PE in the same weekend will set you back about $340. Getting your test results recognized by the State of Illinois will require an additional $100. If you add this to the cost of any review material you might want, getting licensed is about a $600 to $1000 investment (ever so slightly higher if you factor in the cost of an engineering education at an ABET school too, I suppose.).