I've attempted to wade through many, many UML texts. Most have two things in common - they overwhelm you with terminology and/or they generally bore you to tears. Chonoles and Schardt's book was a pleasant surprise. They do a great job of simplifying concepts and diagramming techniques. And best of all, they don't weigh you down with terminology prematurely. They introduce buzzwords and phrases as they're needed in the text.
As for the downside, the only thing I can really complain about is the use of examples in the text. They beat on the same example(s) over and over (I feel like I know everything there is to know about air filters). Repetitive examples like that create a bit of a monotonous tone. But they do in fact get their point across, so it's ok I suppose.
For folks looking for a definitive reference to UML 2, this is not the book. It's an introductory book only. But it's a GREAT introductory book. A true gem for UML neophytes.