blah blah blah - back onto the topic
as for the power inverter or dc-dc power supply, my answer will be it all depends.
i use my car only on weekends and do not commute with it so i dont really have a need to have a power converter installed into my car.
my tablet laptop has around 2.5 hours or more (depending on screen brightness) battery life which is more than enough for the usual around town running around.
when i go on roadtrips, i go with the laptop juiced, and maybe a couple of hours down the road, i use a single plug power inverter and start juicing my laptop from there.
yeah it's noisy but i can't really hear it over the music, and i dont blast my music. it gets the job done, was $50 (i know - i paid too much) but it does the trick.
i feel that if you use your car on a daily basis or you commute in the car for longer periods, then definitely go with the dc-dc solution. just qualify your need for it though - is it absolutely necessary? or do you need it only once in a while like myself?
plus, with the inverter, when my laptop is running off of battery, i can charge my phone or something else like that.
just a thought.