I have two and I think I can heat around 7.5g water from 60s to 180 or 190 in 70-80 minutes in a 10g round cooler with the lid loosely covering it. I thought I could leave just one element plugged in to hold the temperature but it eventually boiled anyway!
I tried using it for an HLT the last time I brewed and it worked well, my only regret (my fault) was not heating the water quite enough to preheat my MLT plus compensate for grain temperature loss at mash in. Don't be afraid to preheat too hot and then just let it cool when you need it to, it is a lot harder to fix it afterward. I should have just stuck one of the elements in my mash tun temporarily.
One concern I have is using it with hard water. They warn you if the deposits get too thick, the element could crack. I don't know if that is a general problem or if I should worry about it, but I won't always be using non-hard water.