It doesn't look like the 1M is meant to replace or actually even compete with the 335i but obviously some folks will cross shop these two models. So far the BMW forums have mixed opine on the 1M with about a 50/50 split from what I read.
To each his/her own, I guess. In my opinion there is limited overlap between this car and the 335is... though it will obviously take a test drive to be sure. I think the official BMW release defined this car best: That really strikes a chord with me. I look on it as the last-gen Z4 M Coupe with at least nominal back seats and just a touch more oooomph.
Car makers try to make a model to suit everyone's taste and budget. What works for one person does not always work for another. Of late niche marketing has helped fill the car makers coffers with high profits for limited edition models such as the 335is, 1M, etc. Porsche must have 2,000 iterations of their basic models and I know there's at least 5,000 versions of the Mustang... Buy what makes you happy.