Yeah, my system is over 3 years old now and I suspect it's about paid for itself already . . . . I got a whopping tax writeoff back when I installed it - I put in new high efficiency windows that same year and I wound up not paying any tax at all!!
I guess the main reason that I went with a direct system is because it was MUCH cheaper and MUCH simpler . . . . and because we have water with next to no mineral content - So why not?? To buy a 'correct' 82 gallon solar tank was just a ton of money and then a bunch more for shipping. I bought all my big parts locally so there were no shipping charges. I don't recall the exact number, but I have a little less than $2500 in the entire system, and after 3 + years of operation, I'm VERY pleased with it - Assuming I'm saving the about the same $50 per month you mentioned, they've already saved me $1800 or so . . . . and I got about a $1500 writeoff, so I'm way ahead already
My tanks are standard water heaters with a full 3 inches of foam insulation. When I was designing the system, I had planned on installing another foot of fiberglass insulation around them . . . . . but, when they are full of 150 degree water, the outside cover of the tank still feels cool to the touch, so they aren't radiating away much heat
To be honest, I don't understand why millions more people don't have solar water heating installed - Of all the 'green' energy concepts out there, it's the most refined and perfected of them all and it also has the quickest payback of about anything else you can do