In addition to the considerations of the level 0 Robes, which I've written a review for, for 2 XP you get the following:
- Cost is reduced by 1
- A icon for your trouble
- "Playing that card does not provoke attacks of opportunity."
That last one is the big ticket item. We've all been in this situation: you didn't get Shrivelling in your opening hand, your apprentice is truant, and then you finally draw it during Upkeep, just for an enemy to spawn on you in the next phase. In the past, you used to dread these situations, especially if that enemy had some other nasty Forced effect if it hits you. Now you just laugh.
Sadly, these robes have the disadvantage of being in the Mystic card pool, which tends to be the most competitive in terms of upgrades. Of course, you can always Delve Too Deep or do a little Arcane Research.
EDIT: The cost reduction is actually also really nice and worth emphasizing. You're not just saving 1 resource on this upgrade--you're lowering the buy-in for a card that saves 1 on the first spell you play per round. That's a lot of savings.