I like the slo mo perk just because it's fun to watch it play out in slow motion. However it does not give a real advantage against enemies since it slows you as well. It slows your draw so an enemy will get to you in the same amount of shots as they would if you were moving full speed. The only difference is that you have more time to aim properly so you are not as likely to miss a shot in a crunch situation. That's the only real benefit.
The trick is to wait until you are fully drawn to hit the slo mo key. Otherwise you will burn all your stamina while you are drawing the bow in slow motion. Just use slo mo when your are aiming for your shot, then release it until you have the bow fully drawn for your next shot.

