I've got the Sunex wobble extensions, they hold up decent. I have had a couple break, 3" in 1/2 & 3/8 drive, warranty was fairly painless. Both had a lot of use. I would recommend a couple spares in those sizes. Summit racing has singles available for a fair price. They will handle quite a bit of torque IME.
I use them a lot for wheel bearing with a low profile socket. Very handy. I tend to use them more then swivel sockets by far.
I fail to understand why more places dont offer them.