My company doesn't really keep hours at all. We have core hours of 10 - 4, and are officially "open" from 9 - 5:30 when reception is manned. But as long as we're around between 10 to 4, we can work our 7.5 hours per day as we see fit.
I generally get into the office at 8:30, and try to leave at 4:00 or 4:30. If I have to stay later than that, or don't get a lunch break, I tend to come in later or knock off earlier sometimes. Just depends on how I'm feeling! Definitely one of the perks of my workplace.