I'm vaping between 4-5 ml/day. I'm using a 5v passthrough (high voltage for me = massive flavor and massive juice usage) attached to a battery power pack and am CONSTANTLY vaping away at the thing. It's not a nicotine issue for me, it's all of these friggin flavors and even at 0 nic, big clouds of vapor makes me happy. I usually vape away at a 12mg/ml solution which for most people is very low but I'm also happy to spend hours vaping 0 nic stuff also. It's still very new to me though, I go totally overboard with all new hobbies.
Easiest no-hassle mod for cartridges is IMHO the B.F.D. mod - great thread on it here
http://www.e-cigarette-forum.com/forum/cartridge-mods/58690-b-f-d-cart-mod.html
I didn't really measure like they did but I've made one of these for all of my cartridges and it's super easy, super cheap and super wonderful. I used a slightly different material but same idea (aquarium filter). It wicks a lot faster than the polyfill that comes in the cartridges.
Burnt taste = dry atomizer. You need to upgrade your carts to the bfd mod. You will get a slightly less flavorful hit right before the burnt taste and you can pop out the dart, drop another 8 drops and be back to happy vaping in no time at all.
Also, juice in the mouth is a flooded cart/attie. Blow the juice out of the attie, stick your modded cartridge back on, wait 5 seconds and vape away.