I'm using these in my two Ultimate 70's that I built from Frank's kit. They bias up just fine, have a greater plate dissipation than el34s, and sound great. I am using them with the adapter sockets right now. You can see them on my gallery. I want to try some Shuguang KT-66's for comparison. If I think it's too close to call, I'm going to rewire the sockets to use the 6BG6GAs. I've written instructions on rewiring the sockets. If anyone is interested I'll post them.
I think that the sound is much more immediate if you know what I mean. Plenty of bass, great separation, etc. I don't notice any power differences, but then again no one would unless measured on a o'scope. Of course only your ears can decide for you. If cost of the tubes are of any concern, then I don't see how you can beat these.