The smoking issue is probably a valve cover gasket leaking. Very common issue on these cars. Fuel mileage could be that it just needs a tune up. One of the first things I do with a used car is replace the plugs, wires, air filter, fuel filter, etc. But it also could have a lot to do with driving style (heavy acceleration, etc) or the type of driving you do (all city vs. all highway).
With regards to the valve cover gasket. It appears that GM has issued a recall now for the front valve cover gasket due to some fire issues with that so you may want to look into that.
