Ok, first off, let's get a couple of things out of the way.
1) Sephiroth is indeed in the movie, from what I've read his entrance is pretty daramatic.
2) The three guys we have seen that look like Sephiroth are called Kadaj, who resembles Sephiroth the most. Loz, the built one with short hair, and Yazoo.
3) Summons are used in the movie, Bahamat unleashes all hell when he's summoned by Kadaj.
4) The children in this movie are a huge part, as well as Clouds guilt trip about Aeries death two years ago.
6) Other characters that appear in the movie are Barret, Red XIII, Cid, and Yuffie, espcially when they take on Bahamat. Also Cloud faces Aeries asking for her forgivness.

The movie was origonally supposed to be 45 minutes long. However, it may very well be four times as long when it's completed. Sadly, it won't be out this year, and is slated to release in Spring '05.