We did this last week and thought it worked out nicely. The view for the parade and the castle projection show were great, and I thought the fireworks were pretty good as well. It would probably be a little better further down main street, but there's just too many people around to move much afterwards.
IMO, if you want to stake out a spot a bit farther down main st. and stay there for it all, that would probably be ideal, but you'd have to do so a good 30-45 minutes prior to the parade (more at busy times). Overall I enjoyed the MSEP FP+ spot and would do it again.