I couldn't really decide which force organisation slot to go for next, and with my new, half painted, Land Raider on my desk, I didn't really want to. My solution to this was simply to turn the page and see what popped up. Elites. Ok.
In the olden days of January, Flamers were probably one of the units, along with their compatriot Screamers, heading the contest for, 'most under costed unit in the game'. Sadly, for all Daemon players who had gone out and bought 27 of them, they are not quite as strong as they used to be, but that's not saying that they aren't a good unit.
The difference between the updated Flamer and the Codex Flamer can be summed up in 3 points. First of all, they have lost Eternal Warrior. This means that they can now suffer instant death when shot at by S8+ weaponry, as they're still Toughness 4.
This change has been pretty controversial, as many if my Daemon playing (other than me) gaming group have been saying that it's fluffy to be Eternal, as they're the essence of a God. My opinion on this is that I don't care if you were created from a God's essence, if I drop a Manticore shell on top of your Flamer of Tzeentch it should die. You already have an invulnerable save for the very reason that you're citing in the fluff, there's no need for Eternal Warrior as well. But what this does is make the Flamers slightly less durable. Against small arms all of the way up to plasma they're just as durable as they ever were, but now a few more weapons are slightly more effective.
Secondly, their Flamer attacks no longer ignore armour saves, having been downgraded to AP4. This was really one of the things that terrified me in the last incarnation. The list that I was using at the time was heavily reliant upon armour saves for its durability, and when you have 54 models (including the Screamers) on the table that ignore them, almost matching my own model count in just your Fast and Elites slots, then I'm in serious trouble.
That said, 6 flamers are still capable of chewing through a Marine squads if your opponent deploys badly for them. If each hits all of the models in a squad of Marines, then that squad is odds on to die. Obviously we're now moving into the realms of your opponent making a mistake which is something that you cannot rely upon. However, a squad of 9 Flamers should do the trick, and they're still extremely durable with their 2 wounds, and considering that not many people are taking too much S8+ at the minute.
Additionally, if you think of backfield objective camping units, you think of Plaguebearers, Scouts, Pathfinders, Gretchin, etc. Flamers will eat through them. Plaguebearers are being wounded on 4s (S4 now) and don't benefit from Shrouded due to flamers ignoring cover. Scouts don't get their cover save or their armour save, same with Pathfinders and Gretchin numbers won't save you as you're wounding on 2s.
Plus, even if there are Gretchin left at the end, how are they going to deal with you? They probably won't have enough to risk a charge through an overwatch, and at that point they're unlikely to be holding their objective, and they're probably not going to kill you in an assault. Therefore, a small squad of 3 Flamers or two is equally viable.
The third and possibly most important change that has been inflicted upon the Flamers is the loss of Daemonic Assault, meaning that you are now unable to Deep Strike in turn 1 and melt stuff. Granted, you still have Deep Strike, and granted, there is less chance of a poor preferred wave roll screwing you over, but when it came off it was absolutely devastating, as your opponent would be less ready for it. More bunched up in their deployment zone, no psychic powers up such as Warp Quake, etc.
However, although it is very apparent that Flamers have taken a pretty big hit in the new Codex, that doesn't mean that they're a bad unit now, as remember how ridiculous they were in the last Codex. In fact, they are still an extremely useful unit. However whether or not they are worth spamming to the max like they were in the last Codex is extremely questionable. So how to lay the unit out. I would probably go for 2 small squads of 3 just to burn small camping units off objectives, however they can still cause a lot of damage to other, larger units too. You don't need to go for 9 to try and eliminate whole squads at a time, leave that to other units. However as a utility unit that's still under 70pts a pop, a thorn in your opponents side, good at knocking them off objectives, and still fast enough and durable enough to score linebreaker, they're a bargain.