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.
I stopped buying U2 records years ago but I was operating under old assumptions. If indeed they are as honest as you claim to be I'll have to give them another listen. I, too can't stand the pap on most Christian music stations. I think most Christians are afraid of being brutally honest with themselves and others in the church about matters of real faith. I don't know why. Somehow I don't think God is easily offended by what our puny minds think up.
If U2 is Christian, more power to them.
If one cannot tell from their music if they are or not, then I'd say that THEY are the ones going out of their way to prevent themselves from being labeled as Christian.
In which case, I'd go with their own desires and say that they are not a Christian group.
I have no problem with rock, rap, soul, etc. being having Christian lyrics and coming from Christian singers. I just think that if you're gonna be one, then don't be coy about it.
Coy to me means that you've not made up your mind.