Hi John
CCD Labs claim the Q Guider will comfortably guide on Mag 12 stars, realistically, I would say possibly Mag 10, your limitation will be the OAG.
You might want to have a look at the screen shots here:
http://www.pbase.com/wjshaheen/root&view=recent
I use a separate guidescope so I am not hindered in any way with what I guide on, I am looking mainly to get rid of the cable tangle and I like the idea of direct output to the autoguider port on my G-11 and GM-8.
At this stage, until I have done some fairly exhaustive work with the Q Guider, the ST-4 will stay put as it is a proven guider, if the Q Guider proves to be it's equal then I will be selling off quite a number of items to fund some other bits.
Cheers
JohnG