"Programming with Libxml2 is like the thrilling embrace of an exotic stranger. It seems to have the potential to fulfill your wildest dreams, but there's a nagging voice somewhere in your head Warning you that you're about to get screwed in the worst way." (a quote by Mark Pilgrim)
Mark Pilgrim was describing in particular the experience a Python programmer has when dealing with libxml2. The default Python bindings of libxml2 are fast, thrilling, powerful, and your code might fail in some horrible way that you really shouldn't have to worry about when writing Python code. lxml combines The Power of libxml2 with the ease of use of Python.