I've reused them twice now.

 

(from the diagram) it doesn't seem to be that much more work to change the lower gasket once the upper parts are removed. So it might be a better idea to go ahead and change the LIM gasket while the engine is apart.

 

I agree with what the others have said, replace the lower if you're doing the upper. I am currently in the middle of replacing my manifold gaskets due to having a stuck lifter. I could tell my gaskets were not too happy when I got them all apart. VERY IMPORTANT: Like someone else has already said- get those two plastic elbows that are part of the heating system. That is my hold up right now and I have to wait till the parts store opens up tomorrow to get them. My car would have been on the road by now had I only known about those little buggers! The whole process is time consuming, especially the first time you do it. Some bolts are in tight spaces and you have to remove an assortment of brackets that are mounted to the engine to have good access.

 

I was lucky that I was able to reuse those little coolant elbows. I got mine back to gether with not to much hassle, but I have taken a few FWD chevys apart before. Make sure you use a torque wrench and the correct tighting sequence when putting the upper and lower intakes back together. I believe the previous owner of mine didn't do that, and thats what caused the intake to be warped.