I got the OptiBrite on my Katzeye for my 7D (but then, i can't compare to a non-Optibrite screen).
with the 70-300L and Kenko 1.4x, at 300/5.6 (420mm f/8 effective), i can MF perfectly fine, it's bright enough for that. But then, so's the stock screen on my EOS 3 (just not as accurate because it's a matte-something). Using stop-down in my best contortionist-impression, the 7D is usable to about f/11 (f/16 effective) and the EOS 3 craps out at f/8 (f/11 effective)

With the Samyang/rokinon 35/1.4, it starts to get a bit darker but still perfectly good enough to MF (i'm pointing out the window on a fairly cloudy midday right now). In fact, it's more than adequate to frame at f/22 (at which point, who needs to focus?).
Compare that to sticking the same lens on my EOS 3, at f/8 it's about as useful as the 7D katzeye+ob at f/22. (eos 3 has whatever screen it came with, anyone know what the default is? I've got an EC-c IV in the post soon anyway)