Ah rain!
Strangely the initial reaction is the noise it creates in the cockpit than the loss of viz - it is noisy - a combination of the thiness and material of the screen and the speed compared with a car.
Viz obviously degrades according to the amount of rain. Heavy rain is sufficient to require you to fly on instruments - not a place for the VFR pilot. Even more rain can be enough to eventually overwhelm the engine.
Heavy rain is associated with CB activity - dont go there unless you really know what you are doing.
Rain will also very quickly find out if you have any leaks in the canopy or doors.
Other than that it will give the airframe a good clean.
Oh and it is wet.