It produces about 29% and 44% less carbon dioxide per joule delivered than oil and coal respectively, and potentially fewer pollutants than other hydrocarbon fuels.
Natural gas is the lesser of evils. (At least as far as direct CO2 emissions go. Damage from fracking is another story.) I'd consider it to be a stopgap fuel until we have a better infrastructure for renewables.
Nuclear does have a big advantage regarding CO2 though. A carbon tax could be a big help with that.