As I'm pretty small, I often find that my view is partially blocked if I'm sitting behind someone tall. Fair enough if it's a restricted view seat, but if it isn't then it can really spoil my experience of the ballet. I once used an inflatable cushion and was angrily told by the person behind me that they're only for children!
I prefer the front row amphitheatre seats with limited legroom as they seem to be pretty good value, but as a treat I like to sit in the orchestra stalls. The one ballet I regretted not seeing from lower down was Woolf Works - I didn't get the full effect of the wave patterns in Act 3 sitting higher up.