I've been using Lydia for around 50 levels. I've got 100 in smithing, enchanting and alchemy, so she is currently rocking Daedric Armor and a Daedric Warhammer. I don't let her wear helmets because she looks nice without them. Overall, I haven't had too many problems. I'm a stealth archer and I just stay in front and shoot anything before they see either of us. I don't usually have a problem with Lydia running past me and getting in the way, although I have shot her in the ass and killed her a few times by misake, but I just reload. If you hug the walls, you can usually guide her past trigger plates so she doesn't set off the traps. Otherwise I just throw a healing spell on her. My steallth is better than hers, so enemies will run past me to attack her. As long as I aim carefully, it's no problem to shoot them in the back. There have been a few times when enemies have gotten behind me, and I'm thinkning, "Dammit, hold still so I can shoot you!" And then Lydia whacks them in the back of the head with the Daedric Warhammer and they go down. Thanks, Lydai!