I am in a 10 man guild, consisting of people who used to be in 25 man guilds and got sick of the drama and politics. We raid 10 man normal/heroic.
Now thanks to flex we can join up with one of the other 10 man guilds on the server - last night we had 22-23 in the raid - to get that 25 man atmosphere without all the crap that comes with the territory, and do content that is still challenging enough to make interesting. Blizzard seem to have got this just right.
In addition to all that, there is another huge added bonus - I never have to run LFR ever again.