Yeah, the UI lag means I can't run Proth Prime Search's cuda32 stuff in the background during the day, even though the cpu stuff runs fine and doesn't affect me.
So, that means probably 10+ hours (min) per day going unused, when if the UI lag didn't happen it could run 24x7.
If there was a way of allocating (say) 95% GPU usage for PrimeGrind, instead of it taking over the whole GPU, that would probably work.
No idea if the way CUDA works allows for that though. Might be an all-or-nothing thing or something.
I'm running a GTX285, but doubt it makes a difference except in the NVIDIA vs ATI aspect of things.