The new Nest update is out and it will definitely help those who have irregular schedules.  With the enhanced auto-away it will detect pretty quickly when you leave (perhaps typical schedule plus detection of outside air temp/humidity when the door opens?).  So now you set it to 70 and then it will detect when you are gone and simply shut down until you come back.  Otherwise with a normal schedule it will arrive at the temp you want at the time you want (dependent on outside weather conditions and it's knowledge of your home's heating/cooling ability.  
That means that right now I have the heat on, and I want the house warmed up when I wake up, so I have it set to 68 at 6am, meaning the furnace kicks in earlier in the morning.  Yesterday it kicked in at 5:50am, Wednesday it kicked in at 5:15am. I have not seen a huge adjustment in my bill, it's about 10-15% down from the same time last year (FWIW) but I am finding the house a whole lot more comfortable now.
It's scheduled to cool down to 62 at 8am on weekdays, but it noticed that the house was empty at 7:30 yesterday, so it started sooner.  By the way, we left at 7:10-7:15am yesterday, so it took 20 minutes to detect, at most.