With IBM I was on 37.5 hours per week, flexi time.
Our department was very lax, we did not have to pre-book our hours, and we weren't even monitored. If you were around when you were needed and you got your work done, that was all that mattered. However, at times this meant a fair bit of free overtime, at other times, we were in the pub Friday lunchtime and then off home. If we did any mad overtime, like staying till 11pm, or giving up a saturday/sunday, we claimed our 1.5, 1.75, or 2x for that.