Of the three Thai WHS that I visited this was my favourite. It is comparable to Ayutthaya, being a former Thai capital now just a pile of glorious ruins. However the ambience of this site was a little nicer, being set in a park dotted with small ponds and lakes, and there also seemed to be more statuary on display. In fact the very elegant and beautiful Buddha statues that Sukothai is famous for really were the undoubted highlight of the place for me, and every set of ruins had a good example.
I used this site as a lunch time stop over on my way between the northern city of Chaing Mai (my personal highlight of Thailand) and Bangkok. It provided a great way to break up the long bus journey, but I think you could perhaps spend an evening there if you wanted to relax and enjoy the site. It is especially good in late November when it becomes one of the best places to see the lanterns of the Loy Krathong celebrations.