Having watched all seasons but season 7, I'll give my thoughts.
Buffy could be very good at times. The arc in the later half of season 2, for example, for very strong, and I liked season 6, unlike most people seem to. But it could also be very weak, and for me there were more weak, or average, areas, than good bits. Season 1 ranged from weak to average. Season 2 started off average and ended very strongly, with great character development and story arcs. The introduction of Faith in season 3 was good, but I just didn't buy her motivations for turning evil, and The Mayor just didn't work for me. It was a pretty average season in total. Brownie points for Anya and Wesley. Season 4 had its moments, but again the arc wasn't very strong; Riley was dull, Adam was a terrible villain and the institute or whatever it was called bored me. The show also didn't feel the same without The Library. This series also began the mostly terrible arc of Spike; his falling in love with Buffy was annoying, unbelievable, and poorly done. Season 5 was mostly good. The introduction of Dawn was very intriguing, and Glory was a good character. The arc worked well. It also began Willow's various arcs with regards to her magic and Tara. A strong season. Season 6 was also very strong, with engrossing yet depressing arcs. However, The Trio were mostly terrible and were given too much screentime.
I find the acting and dialogue in this show very overated by the fans. The dialogue was mostly average, with some especially clumsy bits and some good bits, and the acting really was just average (improving from poor near the beginning). Neither the dialgoue nor the acting can compare to a show like Breaking Bad, for example. I want to take the time to point out the standout episodes, Especially The Body, but also Hush and a few from the particularly strong arcs. There were weak episodes, but for something so long that's bound to happen (there hasn't been a weak Breaking Bad episode yet, but it's much shorter). But average acting, dialogue and episodes as a whole means I can't rate this over a 7. There was very good character development at stages, but a lot of character development was unbelievable, sort of negating the strong bits.
Buffy could sometimes be weak, sometimes be average, sometimes be good, and sometimes border on great. However, I struggle to put the series as whole in the 'green' area of the 7s and above; I'll be generous give it a 7/10. It's just not consistently good enough to rate higher; arcs can be very strong or very weak, and the dialogue throughout the series is only average. In my opinion Buffy is highly overrated, especially compared with TV like Breaking Bad, House, Torchwood (excluding series 1 and 4), Doctor Who, Homeland, Life on Mars /AtA, LOST (first few seasons, at least) etc. I knew it wouldn't live up to its hype, and it absolutely didn't; as a series as a whole, it's good, but not good enough to put it close to the shows above (I'm still not sure whether to give it a 7 or 6.5; 7 seems a little too generous and 6.5 a little too harsh).