From what I was told by a friend of mine who lives in BKK 6 months a year, it's better to fly to Chiang Mai than drive. Drive time roughly half a day, in spite of the seemingly short distance (think road to Hana on Maui).
We spent 3 days in Chiang Mai this summer. It was great. CM is Thailands 2nd largest city. The folks that live there say to go to Chiang Rai for an even better flavor of Thailand. Less congestion. That's roughly 4 hours by car. You can even book overnight tours to C.R. for hardly any $$$.