Love, love, love the bonus stuff. It's great to listen to the extra discs and hear music from the same era that wouldn't make sense on the original albums we all know and love. Some of my favorites so far:
Boy: the U23 EP in good sound quality, and yes, even the alternate, acoustic guitar-heavy mix of I Will Follow
October: A Celebration with great sound, the wealth of live tracks and BBC sessions and even the Common Ground Remix of Tomorrow... actually, October has the best bonus disc of all the reissues so far
War: This one falters a bit, but I agree with Dan Smee that Angels too Tied to the Ground is fantastic. It's nice to have the older 12" remixes in decent quality, but I could live without the modern remixes of NYD.
Unforgettable Fire: This is a close runner-up to October for best bonus disc. Disappearing Act and Yoshino Blossom are great outtakes, the quirky remix of ASOH is interesting (c'mon, having Peter Gabriel sing backing vox on a U2 song is cool... although I do agree that it sounds a bit Lion King-ish in places. Actually, the "ooh way oh way ohs" sound a lot like the Dream Academy's "Life in a Northern Town").
Joshua Tree: It's great to have so much unreleased stuff from the legendary JT sessions, even though it's obvious why none of it turned up on the album. Plus the Edge is correct in saying that this is U2's best set of b-sides.
I'm a sucker for this kind of stuff, but I know it's not everyone's cup of tea...