I've been looking at this myself, yes the initial investment is way more but when you factor the life of the LION batts, the fact that you can discharge them 100%, the weight and space savings,how quickly they charge, when you compare everything "apples to apples" the LION batts are much cheaper per amp hour than lead acid or AGM's. You just have to pay for it upfront, kinda like paying for a diesel vs gas up front.
I am seriously considering them for my TH especially after installing a residential fridge and PSW inverter.
