Greg gives some great tips for using parts libraries in EagleCAD, be they the original CADSoft libs or those provided by others. He writes:
My first recommendation is to not blindly trust library parts created by others. The problem is that I have found way too many errors in libraries created by others. You might think that since someone else has a working design using a part from the library, that the part must be right. While it is a good sign, the reality is they may have corrected an issue in their copy of the library, or worked around it somehow.
So I check any part I use for the first time from another source very carefully. It takes some time to do a thorough check, but it is far less time than you will loose if you have PCB’s built with errors.
I start a design by downloading the datasheets for parts I am going to use in my project. Usually near the top of the datasheet you will find pinout information, and near the bottom you will find the package dimensions and sometimes a recommended footprint.
The manufacturer’s recommended footprint is meant for volume production on automated machinery. I will take it as a starting point, but I usually increase the pad sizes for hand assembly, to give me more pad to touch with the soldering iron. This is especially true for SMT parts.
You can read his whole post here — lots of good advice in there!