We've spent time in San Miguel and have friends who live there. It's a small city (about 90,000 in SMA proper, 180,000 in the municipality) at about 6300 feet - think Santa Fe-style high and dry, without the snow! Very artsy, lots of wealth, and like Santa Fe the downtown area has a very touristy, Disneyland-like feel - but walk 4 blocks in any direction and you're back in real Mexico.
For a single person or younger (below typical retirement age) person it certainly has much more going on than the Lake Chapala (aka "Lago de Geriatrica
) region where we live. There is always something going on: film series, concerts, visual arts, classes, jazz, new high-end restaurants, whatever. Rents and real estate prices are higher than at Lake Chapala, but one can still live very well there on not much more than social security income if one really watches it. There are a lot more tempting things to spend money on though than at the lake.
Here is an excellent web site that's the best portal into life there I know of. Check out the forums, FAQ's, etc.:
Welcome to San Miguel de Allende, Mexico