I've been studying Houdini Apprentice for the past couple of months. I'm far from an expert, but I believe I know enough to start assembling a reel. I've heard you should tailor your reel to the job. If I want to go pro what kinds of jobs are out there using Houdini?

It really depends on what you want to do. FX is obviously what Houdini is known for so getting a good all round knowledge of the different types would be good. In VFX, companies seem to be looking for Water/Fluid TDs but these trends change all the time. More and more companies are integrating Houdini and Mantra into there lighting/rendering pipeline so that's also a viable route. As I said it's mainly down to you and what you enjoy doing.