BIRT is relatively simple to use for simple things, but for complex things it is a bit confusing. The documentation is very poor in certain areas, in particular the API doc is not updated frequently. Do you need to do anything that is not exactly like the standard BIRT examples, aside from page size?
Check what you need to do, as Horcrux7 said -- if you'll need to write a custom extension for a data source/output format, process TIFF images or deal with multiple "datasets" in one report item, you just might lose a few years off of your life-span and grow more bald solving the problem. :-)
BIRT is heavily dependent on Eclipse, and thus quite flaky. However, my experience with Crystal Reports has been somewhat worse, so I give it some slack. Just note that BIRT is not open source in the usual way -- one company is responsible for nearly all there is to it, and there is very little real "development community" to lean on, just any internal people whose supervisor might deign to allow him to throw you a bone. The poor documentation of the source code and occasional bizarre OOP hierarchies leave the beginning obfuscated code contestant speechless. :-)