Is there any reason I shouldn't buy a QHY5L-II?

[codemonkey]

Intended use is for guiding a Celestron CG5-GT, likely with PHD although I'm open to suggestions on software with a preference towards freeware.

[Shiraz]

qhy5l2 with PHD2 and a short fl guidescope is a great combo.

[Amaranthus]

I was also considering this recently, but ended up going with the ASI120MM-S. It has exactly the same sensor as the QHY-5L II, but a larger body (pro: better heat dissipation, con: doesn't fit directly in the focuser tube), and the software seems less fiddly to set up with Metaguide. The biggest win for it, in my book, was that it was USB 3, so the frame rate at 1280x960 is 72fps!

You can use either of these with PHD2, but Metaguide is more suited to their capabilities, with its emphasis on fast centroid AO emulation. I use PHD2 when using my CCD as a guide cam, and will use Metaguide for my new AS120MM-S (when it arrives - currently using a NI5 for that).

[codemonkey]

Thanks guys, I think I'll lock it in :-)

ASI120MM-S sounds good but the 5L-II is cheaper and I'm somewhat restricted there. Don't quite have the funds for an OAG or guide scope to go with it yet either.