There were some abysmal episodes in season 1 (Emancipation, Broca Divide), but I think the show settled quite quickly, much faster than any Trek, with the possible exception of TOS. I tend to think of the last two seasons as a separate show, and while I like Mitchell and Vala, I don't like the Ori storyline much. So rarely for me, I like the stand-alone episodes better than the Ori arc episodes.
I really enjoyed RDA's O'Neill in the first 6 or so seasons. After that he became so cynical and irritating, that for me it was actually a relief to see him go. My biggest gripe with the later seasons was that they had to use devious means to ensure that Carter wasn't promoted to leader of SG-1, which IMO she should have been.