Which bushings? Control arm, rear trailing arm, shock mounts, tranny, diff etc..
I didn't really notice any difference when I swapped to the '95 style M3 offset CA bushings. It wasn't any stiffer but they last a hell of a lot longer. Obviously I had more turn-in because of the offset.
I felt a slight increase in stiffness when I swapped to the M3 rear trailing arm bushings. However, my previous bushings were pretty worn out so I can't really compare the two. They also last longer as they have a metal casing which compresses the bushing when installed.
If you're more performenace oriented, I'd opt for the Urethane bushings. Unless you can find super good deals on M3 parts or find them used, I think it'll be more worth it to go with the Urethane bushings.
Anyways, swapping in M3 parts can only do good things for your car
Some parts are identical to regular E36's though, so becareful what you swap. Some dealer part numbers have different part numbers for M3's and regular 3 series, even though they are the exact same part. They actually charge more for the M3 parts in some cases. eg. M3 rear trailing arm bushings have different part numbers than the 96+ E36 bushings, even though they are the same.