Here's the deal: I wouldn't change an ADR due to bad reviews. I read the bad reviews regarding San Angel and still went. If I had to report back to you concerning San Angel, I would give you a bad review.
But here's the situation- I'm still very glad that I had the experience of dining at San Angel. In other words, everything there is edible (except for the Flan, but maybe I don't like Flan... first time I tried it was there.) Sure, they might take some shortcuts. The food might not be on the same tier of Le Cellier or Coral Reef in terms of quality. And its kinda crowded. And the whole experience seems like you're eating inside an aquarium because everyone in that line for the ride is just standing there... watching you eat. Looking at your food. Checking you out to see your facial expressions for signs of whether or not it is worth eating there themselves.
But guess what? When's the last time you ate volcano-side? When was the last time you eat in the middle of a piazza? And when was the last time that you ate where people look longingly like they want to trade places with you?
In other words, it's an experience. Even if you are not particularly fond of the food, you will be satisfied that you at least tried it.
But let me give you a few pointers to maximize your experience:
1. Don't try to just eat "Mexican" food. In other words, don't go for the burrito, enchiladas, etc. See San Angel doesn't serve ground beef. Therefore, those "Mexican" foods will have a texture and taste that will be different than some of the foods that you will be used to. It ain't Taco Bell, that's for sure. (But if you like that kind of food, San Angel's Cantina is a good place to eat for CS.)
2. For mixed veggies, they use something that is very similar to Veg-All. Remember those canned mixed vegetables- Veg-All? I'm telling you, that's what they use.
3. When they seat you, do not- repeat- DO NOT- allow them to seat you next to the restrooms. It's like restaurant purgatory. There's no ambience. There's no scenery there. It's just folks bumping your table on their way to the restroom. And you might even hear toilets flushing, I don't know. So if they walk you to the restrooms, gently and politely tell them that you will happily wait until they will get you a better table.
If you follow my advice, you might have a great time there. Personally, as someone who's not a big "Mexican" food eater, I ate the ceviche appy with the fish, and I liked my food. (I didn't eat the Veg-all, never liked that stuff.) My eldest DS did basically the same thing and loved it. But my DW and youngest DS are big-time Tex-Mex eaters and they were thoroughly disappointed, so much so that we will not be eating there again. But even they were glad they at least tried it.
So try it. You never know. There have been plenty of bad reviews regarding my favorite restaurant and we just keep going back year after year enjoying it. (Our favorite restaurant, by the way, is Coral Reef. DW loves it! I can basically eat anywhere. I'm flexible.)
The food had come a long ways and gotten significantly better than it had been in the past. I would say that many of the negative reviews posted were based on how it used to be and I wouldn't have any second thoughts about eating there now with their new menu and quality. 
It's one of our favorites, and yes, the atmosphere helps!
