If you've read my last post on the great fuel savings that is Hypermiling...then I must tell you, there ARE down sides to it.
You will be slower. My 17 minute commute has stretched to about 25.
You will not be liked by other commuters. Especially the kind that want to gun it at every green light.
You have to be extra careful about pulling into traffic...since you are SLOW, people can fly up on you very quickly.
You will become obsessed with calculating your new MPG. This will annoy your spouse/children/in-laws.
All in all - as long as you exercise a little extra caution when driving, I still think Hypermiling is the way to go.