From what I've found it's just a sort of extension for NEI.
I quote from the minecraftforum thread:
"I have the pleasure to introduce to you Waila (What Am I Looking At). Waila is an extension for NEI. It will show in the tooltip what mod an item is from. Since the tooltip contains the mod name, you can also easly parse items per mods.
But that's not all ! Leveraging an extended NEI HUD, it is also capable of showing the name of blocks and mods ingame, right on top of your screen.
And if this is not enough, it comes with a few integrated modules to show internal information of some specific blocks. For now, BC tanks have been implemented (shows the content, capacity and amount of fluid) and IC2 machines (shows the In/out voltage)."
When reading that I think it won't be a gameplay changer. Only some NEI features that won't work?
And if I'm right on that it wouldn't be such a problem as I'm still setting up the server.