I try not to see any car as a "step-up" into any other one. Speaking from experience, I will tell you that choosing a car will always be a combination of budget and needs. I am about to place an order for a 2011 M3 ZCP, and I am coming from a 2009 997 Turbo. The last time I checked, I do not feel like I am doing a "step-down" per se. I just have always wanted an M3.
So, my answer to Redduck1's question: It's not a transitional car for me, and I feel like I will likely continue getting M3s going forward until I get bored. I just drive what I like regardless of the $. Drive it because you want to.