1: How long have you been in the hobby?
Since roughly 1998, but it wasn't until 2004 when I bought my first Astartes set. My Father played Warhammer 40k for years before "retiring" (a.k.a selling all his sets) in 2000. He introduced me to the game and helped me grow into the storyline/novel series. It wasn't until 2004 I got into the TT game and I have been an addicted fan ever since.
2: How important is Fluff to you, relative to other aspects of the hobby?
The Fluff, or the body of knowledge especially of a traditional, anecdotal, or popular nature when discussing a particular subject, is the most critical part of any game. Esepcially in games based for Table Top or Fantasy the Fluff is what allows the player to let his imagination run wild as he paints, builds, and fights his ways through simulated Dungeons/War Zones. Thus to me if the Fluff of a particular game is not good, or unappealing, then the game is not worth it's weight in salt to invest in.
3: How devoted are you to the works of the Black Library? By this I basically mean what compels you to buy/read one of their novels, do you pick them because of the races involved, follow certain serials, on a whim, etc...
Certainly the different races, Sci-Fiction setting, Weaponry, and bad assness of some of the Character was appealing, but what caused me to fall in love with Black Library was how well written some of these neovels were. Having been a avid reader for as long as I can remember I always held a love for Science Fiction/Alternative History novels. I started with Star Wars, converted to Star Trek, then got hooked on Harry TurtleDove novels, and many more. I was harsh on many writers, esepcially Fantasy writers; expecting them to weave a great story that captured the imagination only to find that they were severly lacking in the end. In Black Library (specifically in Abnett and Mitchell's works) I found exciting stories with relatively easy-to-understand Fluff that was entertaining for the imagination.
Thus I am a Devotee, Follower, Worhsiper, and borderline Zealot of Black Library. I recognize there are many good writers and Warhammer 40k publisher's out there but in BL I find the stories that I enjoy endlessly flowing to my E-Reader.
4: How many codexes have you taken the time to read thoroughly, including older editions?
Many! I've read all 4th, 5th, and most of the new 6th Edition Codex's. I own the Astartes, Guard, and Ork's latests versions.
5: Do you enjoy postiulating theories, pondering mysteries, and writing your own works to expand your enjoyment of the hobby?
I do, but as for writing my own works.....well it's ongoing. I love creating universes, and setting up the world around the story, but writing the actual sotry is not a strong-point of mine. For example I could blow up the Star Wars Universe long beyond just the MilkyWay Galaxy, but to write a story would be very difficult.
6: If you have anything further to add, feel free.
In reference to the actual question posted as the title of this forum I can say that I have read quite the Lore on all forms of knowledge when dealing with the Warhammer 40k Universe. Whether it's the Novels themselves, Lexicanum, Codex's, DLC's, Add-Ons, or Supplemental Codex's I have kept up on the basic Lore. I confess that I am not an expert in certain fields by choice; the Prime example being the Heresy series as they do not appeal to me in the slightest. However when it comes to the Armies themselves and the development of units, Lore, and Characters I am very much in the Lope.
EDIT: 7: What would you call your area of expertise?
The Guard and Tau. I particularly hold a love for these Races and keep up on every novel and Codex that is put out for them. Following that I'm well versed in the Ork, mainly from playing them so much and developing my own Army's storyline via the 5th Edition Codex.
EDIT8: What would you say is your weakest area of fluff knowledge?
Eldar, I know little about them. Choas as well, though recently I've been starting to read their novels to get a better insight into their.....warped perception.