Lets get down to it, these units have changed drastically. You can call it the nerf bat or you can just say that they are brought back to reality, but the screamer/ flamer spam is long gone. Lets take a look at these units, and their changes.
Screamers. (jetbike unit)
At first glance these guys stayed the same with a very similar stat line. Now being a Daemon of Tzeentch (special rule), screamers get to re-roll all failed saves of 1. This gives them a much better saving throw than they had before, and it means that when they are turbo boosting, they will be getting a 3+ save.
After this is where there are subtle changes that alters the screamers that we loved to see or hated to face. The biggest is Lamprey's Bite
As it stands now in order to use Lamprey's Bite, you must sacrifice all your attacks, in order to use this ability. Its still S5 AP2 with armourbane. So thats the biggest change, in that now, for the same point cost, you only get one Lamprey's Bite.
Slashing Attacks are still the same as well, as well as squad size.
Flamers (jump infantry)
Another Tzeentch unit, they get the same bonus to saves that Screamers get. One again point costs are the same, but nerf bat came in the form of Warpflame. Before I get into that though, the Pyrocaster can take up to 20pts in Daemonic Rewards in any combination. Lessers at 10pts each, and Greater Rewards for 20pts. (As I get more into the codex we can take a closer look at these)
Warpflame got hit hard, and it goes like this. S4 AP4 Template, warpflame. Warpflame is now checked for at the end of any phase in which one or more unsaved wounds occured. Any unit that that has suffered one, now must make a toughness test, and if failed suffers D3 wounds with no armour or cover saves allowed.
Here is the tricky part. If the test is passed, all models in that unit gain feel no pain 6+ for the rest of the game. If they already have it, they gain a +1 to all no feel pain rolls the rest of the game.................
Wow, I think flamers just might make things easier on some armies.
There you have it.... While I think personally that they are brought back a little more into line with what they should have been (minus the feel no pain thing, which is a quirky rule), they have definitely gotten a 6th edition adjustment.
Later I will get into more of what you can expect (of course if you have been following along with the QnA and other info, you should already be on top of it)