New sif is actually quite good, although it probably was effectively a nerf for mummies of sif.
Firstly, sif piety gain has changed slightly. Now, you get 2/3 of the old piety from training spell skills. You also get 1/3 of your piety from kills. What this does is mean you don't HAVE to train spell schools constantly to keep piety up, since you're getting some just for killing. You will need that piety now too, because the new channeling ability uses it, and it's quite good.
There are two new abilities. Old channeling is gone.
New 1* ability: I forget the name, but it's a free, instant activation ability. When it's active, you can cast a spell without enough MP, but you get a -cast debuff somewhat proportional to how much MP you used that you didn't have. Actually much more useful than it sounds, but obviously weaker than old channeling most of the time.
New 2*(or is it 3*?) Channel Magic: Based on invocations skill, and uses some piety, probably some hunger too. You get super MP regen (like 5-7 a turn) for a duration dependent on your invocations skill.
So the way it plays out is a little different than before. Previously, maybe you'd fight some guys, find yourself out of MP, and press 'aa' for a few turns to get enough MP to cast again.
Now, you fight some guys, find yourself out of MP, press "aa" (I think it's still on a), then cast spells, because your MP is regenerating faster than you can spend it.
Alternatively, maybe you just need one more cast to finish the fight, so you turn on your free cast without MP ability, cast a big spell to finish the fight, and the -cast debuff doesn't matter since the fight is over.
I got great use out of the free casting one with a FE, where I'd run out of MP, turn on the free casting ability, put a sticky flame on the enemy, run until the -cast debuff went away, used up my MP, turned the ability on again, and cast another sticky flame. Repeat as necessary. FEs can also take advantage of this ability with conjure flame, since you only need one cast to block a path!
All in all, I think it both makes sif stronger, and much less annoying to play. No more aaaaaaaaaaaa between fights!