full size mudguards if your going to ride in the wet.
most people avoid it anyway though but if your commuting obviously it's a must.
pain to fit but once they are fitted you can take them off and on easy enough if you wanted to for the summer, but as others have said they do offer some benefits in dusty conditions mainly for others behind you though,
no stones getting flicked at peoples faces etc instead they rattle around in the mudguard and fall harmlessly to the floor
if a bike has disk brakes you need to check he callipers wont impede the metal wire stays that hold the mudguard in place, you can on most bikes bend them around the callipers.
preferably you want the rear calliper mounted inside the triangle on the back off the bike like so
as opposed to being in the way like
with what's meant to be a straight metal rod will need bending around it like as above.
same for the front on most too
The above bend examples are from the same frame I have and the reason I haven't bought any guards for it yet because bending the stays is a real pain in the butt
I guess once there on they are fine though
just a bit untidy looking but with that bike bag I guess he doesn't care about looks over function