Since starting 40k in January 2009, I've always loved the fluff and models from the Grey Knights. There is just something made of pure win about knights in shining armor hunting down the things that go bump in the night. I tried to start the army a few times, each time buying more of the metal models, but could never get past one simple fact.... The DH codex is not competitive in 5th Edition.
I understand that quite a few people have done very well with the DH codex in 5th Edition, but I'm just not that pro and I'm too "competitive" to stomach that much losing. So each time I just ended up writing up some lists, playing some test games, and eventually giving up.
However, the new GK codex looks to be pretty solid and have a multitude of competitive builds. My favorite thing about my Codex: Space Marine army is that it is extremely flexible, and I can run a completely different build each week. I think to a lesser extent GKs will be the same way. While they lack the sheer number of unit options of other books (e.g. only two fast attack... seriously?), I think the units they do have are flexible enough to make up for it. Overall I think that GKs will find themselves around Nids or BA in powerlevel, which makes me happy. I won't be tempted to play the martyr card when I lose, and I really can't be accused of being a codex-jumpy powergamer if/when I win.
I'm taking this codex release as an opportunity to start a new army that is competitive and has great conversion/painting potential. By the time I finished painting my Smurfs, I had learned a significant amount about the hobby aspect of 40k and my latest models are light-years ahead of my first ones. That and having a fully-painted army already removes the temptation to rush and speed paint models just to get them done. I can actually take my time on each model and really make them look good... or as good as I can anyway.
Possible lists and paint schemes incoming...