I've used a 25 mile 6v Parmak for about 3 years now. It is on a pretty short run, probably around 5000 total feet of hot wire. Puts out a good 5800-6000 volts. Will drop without sun for 2-3 days, but generally any decent sun in a day will keep it charged. Had a lightning strike a year ago and ran to town, bought the only thing I could find on a Sunday which was a 3 mile 6v Zareba at TSC. Used it for about 2 weeks until the other one was repaired and didn't have any problems with it. It did run a bit lower...something around 4500 volts if I remember right, but it's a smaller charger so that wasn't a surprise. As a side note, Parmak did honor their warranty and fixed my charger at no cost to me other than mailing it back. I'm sure other reputable brands offer similar warranties, never looked into it though.
As they said, grounding is definitely important. I have 5 ground rods total, 3 at the charger and 2 on opposite ends of the fence. Had to put water to them during the worst of our drought in 2011, but other than that no issues. It will tolerate some weeds on it, but I try to keep the line sprayed to keep it clear.
I would assume you would be better served with a 12v charger, but I'm a pretty low budget operation and have no complaints with my 6v. Cows and bull have respected it so far. Small calves will slip through at times, but they can get through concrete as far as I can tell.
