Is Land Travel from Bangkok to Chiang Mai a Good Idea ?

No experience with Circle of Asia, however in answer to your headline, why not hop a train from Bangkok to Chiang Mai and return by air. I caught a train around 2300 hrs ex Hualamphong Railway Station in Bangkok and arrived in Chiang Mai next morning around 1100 hrs. Comfy bed, 2nd class with fan and lower bunk around 20US. Buy ticket one or two days before at station or from agent paying around 2US commission fee.

Met many Thais and plenty of foreigners. Take some snacks, however food and beer is available. Take a luggage lock & secure your bags to luggage rails close to your bunk. Lots of local travel agents who can organise a look-around for you. Or hire a car and drive yourself. Roads have improved since 2004 when I drove the circle route from Chaing Mai via Pai & Mae Hong Son, Scenery lush with many places to stay. Dec/Jan it's quite cool up there. Have fun!

I was principally interested in the historic sights along the way (Sukhothai etc.)

I think they are rushing the activities on day 3 with only 1/2 a day allotted for the visit to both the Sukhothai and Sri Satchanalai historical parks. The drive between the two parks will take around an hour already and you'll not have that much time to explore both places.

Apart from that you will get to visit some highly revered Wats and will get to visit towns that most tourist don't go to.

Here are some of my pictures from Sukhothai and Sri Satchanalai if you're interested: http://khun-pook.smugmug.com/History/Sukhothai-and-Sri-Satchanalai/6915625_wsTws

I did a similar trip on my first visit to Thailand 20 odd years ago with a car and driver and really enjoyed it (of course, thing have changed a little since then. As Hanuman points out, parts of the trip do seem a bit rushed. I do remember getting on a slow boat from BKK to Bang Pa In took a couple of hours extra but was a great alternative to sitting in a car and the time in Sukothai does seem very short. I visited a second time 4 years ago rented a self drive car for a week in BKK and dropped off in Chiang Mai. This gave us much more flexibility e.g. we stayed overnight in Sukothai and rented a couple of bikes for an early morning ride around the complex - a great time for photos as there are less people about. Again, as Hanuman points out the best part for me was getting to see towns where few tourists go and just wandering around the markets and temple etc. Have you investigated the cost of a private tour with guide?

OK, suppose I fly to Sukhothai and stay overnight to do justice to the historical park. What then is the best way to get to Chiang Mai from there?