I actually thought Season 9 would have been a good point to end it.
Season 10 hasn't been bad, but without actually showing too much of Darkseid (unlike Zod, the villain last season) I don't think the show has managed to portray this feeling of dread which we're obviously meant to be having about his impending arrival. There's also been a lot of filler episodes this season which don't seem to have progressed things a lot.
That sinking budget isn't helping things either. I watched a few episodes of Season 1 of Smallville last week, and the difference in the look and feel of the show is startling : I hadn't even realised how different it is now until I watched them. In early Smallville seasons you really felt that there was a big world out there beyond Smallville; loads of different locations, wide open roads, outdoor scenes at forests/lakes/etc, and Metropolis actually looked like a city. Now we get the same sets rehashed over and over; interior shots are always carried out somewhere with faux light spilling in the windows and you never see any real location or movement through them windows (and that golden sheen over everything in the Kent farm house is starting to bug me bigtime); street shots are always done on the same street, but from different angles; and large 'crowds' of people tend to involve 5-6 random extras.
I still enjoy the show, don't get me wrong, but I think it's the right time to wrap up - the budgetary limitations are really draining Smallville of some of it's lustre.