I intend to rent an apartment in San Miguel next year and probably spend about eight months there. Hands down, it's one of the best cities I've ever been to. I miss it!
San Miguel de Allende is amazing. Its colonial architecture and old buildings are just a living painting, full of colors. The market and the church are the best places to visit.
San Miguel de Allende is a great colonial town, unique because of its large artist population, and galleries galore. There is a lot to see right in the city center, and amazing hot springs (La Gruta is very neat - with a cave) just outside. There are a lot of restaurants with very good food (and desserts...), and the plaza is always lively.