As mentioned, in my view this is much better than Might of the Ancients. You lose some strength against vehicles in close combat but with a Force Staff you get another +2 Strength giving you good chances of penetrating anything short of AV14 (which is pretty hard with S6+2d6 as well). More importantly, you get +D3 Toughness to boot and Eternal Warrior, making your Librarian very durable (remember blessings last until the end of your opponent's next turn!). I used to target weak squads with my Librarian in the last few turns, and that was very risky with two Wounds, but with Iron Arm the grunts will have a much harder time hurting him. If you went for the Terminator Armour and Storm Shield upgrade, the Librarian turns into an absolute monster. Watch out for those Powerfists though, even with T7 they can chip away those two Wounds quite easily. Challenge them and kill them before they strike or force them to draw back from the fight.
Anything that modifies Toughness on a squad-level is totally awesome, right? Wait, it reduces Strength too? And puts you in difficult terrain? This is by far one of my favorite psychic powers ever, right up there with Null Zone. This power lets you counter tank units like Nob Bikers or Plague Marines by robbing them of their defenses. Since Instant Death is now resolved after modifiers, you're instant-killing T5 Nob Bikers with S8 again, just like old times. And they're not getting Feel no Pain anymore as well. Let fly those Krak Missiles!
At first I wasn't too impressed by this one. Relentless is nice I guess, but how often do you really need it? Feel no Pain is alright, but It Will Not Die needs some luck to even pay off, so you're left with Feel no Pain. But wait, you can bless vehicles with this! This means you can cast Endurance on a Vindicator or Predator, move 12", and fire as if you were stationary. Not huge, but already a bit more useful. Feel no Pain of course gives a decent defensive boost to any unit in need, with Assault Terminators being an obvious target to boost their defensive capabilities even further. Scoring squads hiding in decent cover (Stealth Scouts, or squads in fortified Ruins) can also get a second layer of life insurance to keep hold of an objective in the later turns. You'll notice that,with vehicles unable to claim objectives in most missions, shooting scoring units off objectives is often necessary as denial units are unable to reach enemy-held positions. Boosting survivability for scoring units is therefore much more important in 6th than in 5th, where scoring units were simply shoved off objectives by Tank Shocks.
I'm not really sure which is better, Life Leech or Smite. Life Leech has the higher Strength and chance of absorbing a Wound, but Smite has more shots. Both are, in my opinion, rather poor Witchfire powers for Space Marine Librarians. Regaining Wounds when you only have two to start with is only useful if you've used your Librarian to soak up Wounds for your squad and failed your save, and you can Look out Sir! for that now. When your Librarian is forced to take Wounds by being singled out, he'll lose two as easily as he'll lose one unless you have the Terminator Armour/Storm Shield set-up. I won't say Life Leech is useless, but I think its use is rarely crucial to your chances for victory, and with a support Psyker, you want his powers to contribute to the greater scheme and not his own survival.
Another combat booster and again a big improvement on the Quickening in my opinion. You lose the Initiative 10, but gain +D3 Attacks. If you could use your boosted Initative for Sweeping Advances, now that would be very nice, but sadly you can't. Still, Initiative 4 is a big disadvantage the Librarian has when it comes to close combat, and buffing it can discourage plucky Power Weapon Sergeants to challenge you and hope to get lucky. However, boosted Initiative alone will not make the Librarian fearsome in close combat. The additional Attacks don't make him more powerful but certainly increase his damage output. Fleet only comes into play when your Librarian is on his own and this is only a good idea near the end of the game, when you're trying to engage as many enemy units as possible. It can on occasion save your butt by giving you a reroll on your charge range but those situations are extremely rare. A decent power but I much prefer Iron Arm.
The last power of the list, and not very strong I think. Focussed Witchfire powers are limited in their capacity as sniper attacks because you have to pass your test by 5 or less. To make matters worse, this power requires a model to pass a Toughness Test, and does not ignore Invulnerable Saves. While it can jump from model to model, unless your opponent is very unlucky this power will claim at most two or three T3 grunts before it peters out. With a meagre range of 12", you're better off with Smite or the Avenger in my opinion.
So let's look at how well these powers match up to each other:
Iron Arm: Good
Enfeeble: Very Good
Life Leech: Average
Warp Speed: Average
Overall I'd rate the Biomancy table as a good psychic table to generate powers from. Remember, you can generate powers from mulitple tabels, so if you roll Enfeeble straight away, you may want to look at the other tables for your second power. Both Telekinesis and Telepathy have pretty decent Primaris Powers in addition to a couple of good random ones, so they're safe bets.