I have nothing but great things to say about my Burris Fastfire III 3moa dot.
You bring up sights being too heavy, the Fastfire weighs 0.9oz. The automatic brightness adjustment is great. You can look through it and see it adjust as you move the sight from a darker place to a lighter one. There are also settings for high, medium, and low brightness.
It is waterproof enough to be submerged. I have not had to change the battery yet, and from what I understand I probably won't have to replace it for a long time. It comes with a nice protective cover that slips over it.
Overall 100% pleased with the Burris Fastfire