Ok where to start. I purchased the blu ray edition in the hope that maybe, just maybe,  I wouldn't feel slightly cheaped out, and slightly dissapointed with the movie anymore. I did enjoy the movie originally, but it was missing something. Would the extra 30 mins of footage make any difference? Heres the verdict.
Yes, I Thoroughly enjoyed the movie this time.
It feels like you actually get a proper ending.
More interaction with other characters in the story, like Yuffie, Cid, Barret etc, not tons more, but enough to make it feel like they actually have a role in the movie, rather than just cameo appearances.
You get more filler story, which helps you understand things alot more, this is especially good for those that haven't played the game.
All in all I was very happy I paid for the movie again, and in my opinion, the extra 30 mins of movie time gives it that something extra.

Personally, I'm kind of sick of seeing copies of Advent Children for sale everywhere I go. As a product, it's something that has easily run its course and SE would do good to finally let it alone. Not saying having it on Blu-Ray is a bad thing - it's pretty expected - but I'm sure it's going to find it's way onto the format in another, even more enhanced/redundant configuration at some later point - as does everything.

AC is something I might buy out of the $5 DVD bin, but even then I would be pretty of apprehensive of buying it. Seems like the kind of thing that might sit there and gather dust - and really, I don't even dislike the movie despite the negative connotation seen throughout this response.

So, they added the 'plot' which was missing from the movie? Maybe I should check it then, I can imagine worse ways to waste few hours.

Great to hear.  I've yet to pick up the BR myself, but I'll definitely be giving it a go now.

Question: was there new music composed for those extra thirty minutes?

Yes, there was. There was an EP released on iTunes (vgmdb). Not sure if that's all the new music though. There's also a 'Complete' version of the OST coming out (S-E).

The new music isn't that good.

Are you saying that based on seeing the movie or listening to the extended tracks?

There's a shocker... not!

Pretty much a big middle finger to those who already bought the soundtrack, right?

No. It's not.

The digital EP that was released didn't have all the new music. This will. The AC Complete OST is named "Reunion Tracks." It will have all the new music, plus likely some "best of" the two disc OST from a few years back, as well as (maybe?) some unused/new-arrangement stuff.

It's not "full OST and then some" so that the old OST is a waste of space. As evidence, take note: the new album is one disc. I think that about sums it up.

Ramza

And I'm not surprised, either.