Superb. The material is presented in the clearest and cleanest way possible, with certain examples brought up again and again to drive home important points. And the treatment of spectral sequences is fantastic!
The material in here is probably standard fare for graduate students, but I think the real benefit of reading this book is the way it gets you to think about differential forms, cohomology, fiber bundles, long exact sequences, characteristic classes, Poincare duality, .... If I had to complain, I would say that a few more pictures would've been nice. But honestly, the explanations are so easy-to-follow, not many pictures are needed!
I just read another reviewer's post, and he/she mentioned the de Rham - Cech double complex. I almost forgot the masterful treatment of this wonderful object! It comes up continuously, and the amount of results and ideas Bott and Tu manage to wring from it is mindblowing (and enlightening).