done it a few times with only lycra – didnt enjoy it as ended up with soggy botty….
used to wear endura singletrack shorts over the top, but didnt like to wash them that much as i love the shorts…
so gone to lycra with gorebikewear alp x waterproof shorts over the top…
perfect combo, super light soft shell, keeps wind off legs, any mud after the ride generally brushes off easily with towel, and whats more you can bung them in the washer with no special proofers/cleaners etc etc and as they are that light they dry super super super quick….
if its absolutly lashing it it down, i go for my dhb event waterproof trousers….again a big pain to wash and reproof though