10-25 man raid finder is correct as far as I know.
I think it could be really good, just so long as you don't HAVE to queue and find up to 25 players. A group of 10 casual playing friends could queue and do 10 man raid finder and opt out of finding any of the other 15.. If that's the case. Casuals will be able to form a nice bond and a group, so will friends and family etc - ready to jump into Normal (currently flex) mode.
What they said during blizzcon is that it will be flexible incase someone leave lfr midraid, then it will scale down from 25 to 24 or whatever number is left of the raid and scale up when someone joins again. So my guess it will first search for 25players to start and then go flexible when people leave.
Yeah, LFR will be flexible, but groups of friends wont be queing to do LFR, its not really aimed at organised groups in WoD. You don't need the gear from it, dungeons and heroic 5 mans will get you geared enough for normals (currently flex). You have to think of it all in context of all the changes happening in WoD, rather than think of LFR and Flex as they are today because they wont be the the same or necessarily targetted at the same demographics in WoD as they are now.