Indeed!
My benchmark is: build Servo, run touch components/style/lib.rs (so nothing other than a timestamp actually changes), then build again. With -C codegen-units=4 on a fast machine, this second build takes 147 seconds. With -C codegen-units=4 and CARGO_INCREMENTAL=1 (for both builds) on the same machine, the second build takes 108 seconds.
So incremental compilation cuts 26% of the time. On one hand this is significant, on the other hand I kind of hoped for more since no source code has actually changed.