1 word: Fantastic!
Personally, I felt that this episode was the best of season 6. I felt compelled to watch every move made by the unsub (played by the young Ben Linus from lost). It was a unique choice to have a thirteen year old kid to be an unsub (last i recall Bogeyman from season 2 episode 6) and this time, they struck gold. Great choice to focus on the unsub instead of the BAU this time and I would say it worked out pretty well. 3 scenes that I loved the most:
1) The unsub acting as a tortured soul where no matter how much one hates that fact that he is killing, one would definitely sympathise with him.
2) The unsub holding the woman on the car hostage. Somehow I had a feeling that he would release her yet he stabbed her all of a sudden. It was a fantastic and compelling scene that shows why I love this show so much: It throws you the unexpected
3) The last scene where the unsub holds his younger sister hostage. I felt that scene was the best scene as all of his anger and hatred for his mother could be seen from that. The hatred and everything threw up and he nearly blew it before the BAU arrived.
However, watching the last part about Ellie uniting with her mother was torturous. It was as if one was watching a cheap love movie. It was definitely uncalled for and we could definitely do without her character interfering with the play. Also, that unsub did not look thirteen at all. Perhaps the writers could have given him a better age to work with (like 16 or 17).
Lastly, it's a great idea to have this unsub to come back again in say 5 years time and be bent on revenge on his parents. It would be fantastic to watch such a continuity. It would give all of us something to look forward to in the near episodes.
Overall: Fantastic episode, best in season 6. And the last episode of season 5 (which was crap)