You know what makes me work even harder in a coffee shop? Leave your charger at home. You suddenly become more efficient since you know your time is limited. Assuming you're not on a thinkpad with a 9 cell battery.
I guess social pressure won out in the end when even people close to me thought I wasn't doing anything. So finding myself in a similar situation to then, I stay for a few more hours (a little over the length of a charge and recharging to full; I don't take lunch breaks myself). I probably am more productive overall, but there's lots of factors to take into account (hopefully I got more skilled) and I'd say the biggest change is the more creative processes occuring in situ rather than strewn about the day. I also check my e-mails a little more often now ;-).