I play both this and WoW (yeah yeah, send your clever WoW insults to the recycle bin), and I was very worrisome about the flex system. In reality, this is how it turned out.

Our static raid team was running the raids in WoW. We had exactly 10 people on the team. (WoW raids are 10 or 25 people.) When the holidays came around with the flex system, we had a few people bow out for some family time, while others were off from School/College/Uni and had the time. The flex system let us take those people in, with normally the group ending up with 13-17 people, even with 3-4 regulars missing. The Content scaled up or down depending on how many people you had in the raid. The content designed difficulty was harder than the "Looking for Raid" system (similar to CT, large group of people who probably don't know each other and about the same difficulty), but not as difficult as a "normal" raid. Their loot rewards were also "in-between" the 2 tiers, so in FF speak, think of about item level 85 stuff.

This system let us raid the end game stuff, with the number of people that we wanted, but without the chaos of what a "Looking-for-raid" group would have, plus our rewards were better. When the team was able to take care of everything and resume normal mode, 10 man raiding again, we were able to take the experience from the "flex raids" which were closer in mechanics and difficulty than the "Looking-for-raid" groups, we were able to plow through the normal mode raids. Now, the idea that we were more prepared for the raid encounters or the overall lack of difficulty in WoW raids are up for debate in another thread, but the fact remains, by using a flex-raid system, our core group of friends were able to continue raiding, but were able to add in a few extra people of the team, and the game was able to dynamically raise or lower the health, damage, and mechanics of each fight to fit our group size.

The hard core raiders, doing the top tier content all the time did not use flex as a raiding mechanism, because they didn't need it for progression. For the people who were playing catch up, or those that had more people than what the raid allowed, it was a far better alternative that having players "warm the bench".

To complicate things in the WoW-world, the talk is to change the raids in normal and heroic (their word for Hard Modes), to use this flex system at the normal and heroic difficulty. Their extreme or "Mythic" mode will be a static 20 when this change goes in. Time will tell on how things go with this flexible raid size as the normal mode raid, but I'm on the side where it may work out as being the MMO standard moving forward, reserving the top tier content for a static size. I disagree with the idea that it's impossible to figure out the variables to challenge the raid on a variable size. We have computers and the study of algorithms to do that for us.