they're a great tool when used as intended (as most tools are). as was mentioned, they aren't a replacement for a corded saw, but a compliment to it. you can quickly and easily get into areas without dragging cords, and they are more compact. they do fine for cross cuts in dimensional lumber, and will generally do a lot on one battery charge.
what i found to be the weak spot is trying to rip lumber. we were pouring concrete at a site with no power. the forms were just constructed on site with cordless tools and paslode nailers. i had to rip a 2x6 into a long tapered wedge and the battery powered circular saw did not like it at all. i assume it was the wood squeezing the blade and fighting the motor. you can't expect a small cordless tool to have a motor like a 15a circular saw, otherwise you'd have to use something the size of a motorcycle battery to power it, and then it's not exactly practical anymore.
as far as lithium batteries go, my experience with them is that they don't work when they get below freezing, but once the temperature warms up they will work again, as opposed to other types of batteries that will freeze and become non-functional. to date, my experience with battery powered tools is that when the batteries fail it's usually as cheap to buy a new tool with two batteries as it is to buy two batteries on their own. if your saw batteries fail, it is probably cheaper to buy a drill with two batteries that will also fit the saw.
