I'm not sure I can offer you much guidance/input as we're just starting to make the same transition.
We're starting to use logged time to right-size planned hours in our templates and to get a better gauge of our resource capacity (actual hours isn't a particularly helpful metric for us--I've noticed that if a user marks a task as complete without first clicking on 'Start Task,' no actual hours are recorded).
One of my colleagues has been running regular reports (monthly, I think) to identify users who are completing tasks without logging hours for those tasks (and asking those folks to go back and add their best guesses as to the time). I think we've seen better adoption (and smaller reports) over the last few months.
There's been some minor grumbling, but we've been pretty upfront with folks that this is part of an effort to make sure we have the right amount of time and staff allotted to work (rather than looking over their shoulders; we also don't do time cards in WF--those are in a separate system).