The Pony POV series, in my opinion, is one of the best MLP:FIM fics around today; it's also one of the longest, over 100 chapters at the time of this review's writing. One of its biggest triumphs is the way it introduces lots of depth not just to FIM, but to the often-reviled previous generations.
The Alicorn/Draconequus pantheon and the way the series handles Nightmares (similarly to how Puella Magi Madoka Magica
handles Witches) help this a lot. One of the best parts of the increased depth is the balance between darkness and the show's original tone; it explores darker themes without throwing the show out the window, and avoids making Equestria yet another Crap Saccharine World
That said, there are some less-positive points. Mostly in the earlier chapters, there are many small spelling and grammar errors. Wording gets awkward at times, even clunky. Some may not like the way Season 6 is a mix of Shining Armor's prequel story and an entire Dark World series. People who don't like long stories will probably be spooked by the sheer volume of the fic as well.
All in all, the Pony POV series is a fantastic fic that explores and deepens the setting and characters, and readers who can ignore its small issues in presentation will enjoy its compelling plot and emotional content.