Well since they're both Adobe products, they're very compatible. Photoshop is a Raster application which uses pixels & Illustrator is vector based, which makes it "resolution independant". You can resize without distortion. Actually create nice brushlike blends & gradients like in PS using the gradient mesh! It takes a while to master, but it's worth it if you don't have both Illus. & PS. I usually create in Ill. & add special fx in PS.