The data I've seen indicate that the take-up on 40-year mortgages was high. Most new first-time homebuyers use whatever is the longest amortization available. For a period of time, mortgage insurers were offering 40-year amortizations, and most new homebuyers were out at that end. When the government imposed a maximum 35-year amortization, most first-time homebuyers were again at the limit.
What we've also seen, though, is that most mortgage holders accelerate their payments through bi-weekly payments or payments over the course of the mortgage that actually move their actual amortization well off the limit.