The reason I said I would downgrade it is because I do think its a great feat to use. If you were invisible and started casting spells on something that was stationary most of the time..you could really scew them over.
I understand that your never guaranteed to find a creature that sits still, but say you multi-class with rogue and sneak up on some monsters or a really high-level one. You could cast many spells and have them all attack on the "surprise" round, while you cast your regular spell. This would also include any quicken spells. I dont see why people wouldnt take this feat, but I guess its just up to people's personal styles. I use this feat a lot and haven't had too many problems with casting multiple spells before the enemy realizes(except for a few Listen checks).
As i said before i think this feat gives you a lot, maybe not in flash and bang with more damage.