I'd call them blatently honest. They talk about their struggles for faith in ways that the Christian music doesn't dare. Look at their two most recent albums - All That You Can't Leave Behind and How to Dismantle an Atomic Bomb. The Christian imagery - and raw intensity - blows past most of the saccharine crap played on Christian radio stations. A lot of their songs are blatently explicit examinations of faith in God in the light of a horribly screwed up world (e.g. "Peace on Earth" or "When I look at the World"). About the only place you find that kind of refreshing honesty - where people are willing to admit to God that they're baffled at how bad things are in the world - is in the Psalms (e.g. Ps. 73). There aren't very many artists on Christian radio who sing above the level of "Jesus-is-my-boyfriend" crap.
Now, I don't know about the spiritual beliefs of U2. It sure seems like Bono gets it - he understands the concept that God operates through grace, and not karma (see, e.g., Grace). At the end of the day, however, where Bono stands is between him and God. I know, myself, I've found his music to be quite uplifting and worth listening to while I run.