It works a lot better for me than what some of you are describing. My character has 100 in sneak and all the perks and when I drink the potion/cast the spell I am gone. With 'Shadow Warrior' it's almost like cheating.
This is pretty much how it is for me. I've got 100 Sneak and all the perks in that tree as well as Quiet Casting. No one catches me unless I stay in the light for more than 20 seconds. No Fortify Sneak enchants as of right now, though my char's a vampire so she gets 25% more sneak.