Just like anything else, as soon as they become readily available they will depreciate like any other car. Look at the Z06, they were selling for 100K, not you can find them for under 60K with less than 10K miles.
I think by the time I'm done with the 335I, all the people waiting on their orders would've gotten theirs and now they will be sitting on the lot, ready to be picked up. It is a very great car, but why buy new if you're going mod everything on it? Buy a used one, when it's out of warranty.
But I have to tell you, I saw the new M3 when I was in Munic for my ED and that car looks incredible in person, it might not be as fast, but for potentially 10K less money it is an absolutely amazing car. I took some pictures of it and every now and again still sit and stare at it in amazement.