Well since no one has answered, I'll take a crack at it. Mind you, I'm not an expert. I've just done some research, as I too have dinner ressies hoping to catch a good spot for Illuminations. So here goes...
No, this isn't San Angel Inn. That restaurant is still there. La Hacienda is a newly constructed restaurant.
From what I understand, MOST of the restaurant has a good view of Iluminations. The closer to the window, obviously the better. Otherwise the highest fireworks get a bit cut off. There is a smaller room that has basically no view of the fireworks. But from one report, if you are sitting there you can leave your seats during the show to go stand by the windows (some windows, somewhere apparently.) The management not only allows it, but assists you in the process. (I only saw one person report this. So I hope it's true).
I'm hoping it will be worth it for a nice entertaining dinner, and not having to fight the crowds to watch Illuminations. I guess we'll all see.
Hope that helps.