I have watched many episodes from the beginning of the series, not when they aired but over several months last year when I was unemployed. I didn't see all of them and stopped watching shortly after the new interns were chosen (13 and the rest) because I found the original interns much more interesting and it was disappointing not having them around full time anymore. I've seen a few later ones here and there.
I will likely tune into the finale tonight, mostly because all my other shows have ended for the season and I have nothing else to watch. It should be interesting.