Just finished Legion today and I must say that I loved the little twist dealing with Alpharius. Gave me a whole new outlook on the primarchs, what might have happened to one or both of the ones expunged from Imperial records, and the death of Alpharius after the heresy.
Flights always been my favorite though, since it is focused more on a handful of astartes who were forced to watch as everything they held dear was destroyed by those they called brother and leader. (The fact that it is written well is a bonus considering who the author is.)
I still think that people knock Descent of Angels when they shouldn't though. Its far from the best in the series because it has little to do with the Heresy itself, but the important story of the Dark Angels occurs after the heresy, what leads to it occurs beforehand and has to be done now instead of later.
Fulgrim and GIF are tied for me as the saddest books in the series so far; GIF because you knew what was coming and hoped it wouldn't, watching marines stand up to their own primarchs and overwhelming numbers that they would never hope to defeat. Fulgrim is just as sad because the entire novel is about the gradual fall of an entire legion and fleet, and then how far Fulgrim is driven by his own corruption.
I'm not entirely certain how many books they plan on coming out with, but I do remember reading that there will be one about the Blood Angels on Calth, and the first signs of the black rage that plagues them now; one about the fall of Prospero and the damnation of the Thousand Sons; one pertaining to the trap created by the Iron Warriors to destroy the Imperial Fists fleet in an attempt to cripple a fourth legion; and at least three books for the siege of terra. (With that one, I personally cannot wait for the defense of the eternity gate, with Sanginius alone holding back the forces of chaos as everyone else withdraws, and then see him get some revenge on the daemon that broke his back on Calth...