I dislike it as they use chemical fertilizers rather than organics. I've noticed NUMEROUS times in my regular houseplants, a white crystaline formation on top of the soil surface. This is the nutrient salts rising to the top and when the water evaporates, it stays behind. RO water helps but doesn't eliminate. I've since switched to Roots Organic potting soil, and while slightly more expensive, I get none of this shit on top and my plants have thanked me ten-fold.
That being said, Gordon Graham grew the world record tomato (7lb 9oz) on miracle grow in 1984 I believe, and despite the Giant Pumpkin world record being broken every single year on various different natural fertilizer/microbe/mychorrizae combos, his is YET to be beat! Miracle Grow actually paid him to travel the states with a replica of his tomato and explain his process. Imagine... 8lb buds....
I personally think Miracle grow sucks ass, but I believe the overall dislike is because you can't adjust to what you want to do when the soil already has everything in it. Also makes it seem like it's for people who don't know how to grow anything... dummy-proof if you will.... even if you won't?