Tau: I had a
SM box in my hand and was ready to buy them. I took a last look at everything on the shelf and saw the Hammerhead Tank. I put that box down and bought Tau instead. I never regretted it.
Necrons: Sometimes, you just hate the world and want to kill in silence. The fluff appealed to me greatly, and the soulless, anonymous mass of killers appealed to me. The models were easy as hell to paint (my first fully painted army) and they looked sharp. Had a great back story, too, with a Martian basing scheme (I expected that the next army book was going to release rules for the Void Dragon and the Outsider. HA! Oops.) Fell on hard times and had to sell the army. The person i sold it to in turn sold it on Ebay less than a month before the new book landed. 4k points for $300. Fully painted. I warned him the book would be out soon, but he did not listen...
Space Marines: The 4e rule book allowed me to make my own chapter with it's own distinctive rules. I made "The Brotherhood of Steel" in true Hipster fashion (before it was cool) and went on to have a nearly flawless record with them. 5th ed arrived and with it a new Marine book (immediately, of course.) It scrapped my concept, redid fluff, railroaded me into one design philosphy, and so I have not touched the army since.
Dark Eldar: Who can resist the allure of pretty tanks? New models, new rules, a strong, yet by all accounts very balanced list.... Yep. I think I'll enjoy them. They do everything my Tau do, only better. (Except survive.

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