Oooh... I heard that too, and immediately did research before I tried it because it seemed like one of those "too good to be true and if it was true why aren't all the farmers using it?"
This was really the best metaphor that I found and pretty much summed up the reason you don't do it
Miracle grow is fine for flowers, but a poor addition to a vegetable plant. It is equivalent to giving a two year old a pound of sugar to eat. The child is hyper and full of energy for two hours and bam, falls asleep, exausted. Miracle grow is too high in nitrogen and makes the plant mature too fast and makes the sexual cycle, or polination time shorter.
I found that quote here (http://wiki.answers.com/Q/Can_Miracle_G ... ose_flavor
) although if you do a simple search for "miracle grow and vegetables" you'll pull up much more scientific and fact-based research on the subject if you prefer.
Since vegetable plants are harvested because
of the fruits of their pollination efforts, vegetable gardeners want to lengthen that cycle rather than speed it up.
My mother-in-law uses a special Miracle Gro made just for tomato plants on her tomatoes in her Topsy Turvy planters and swears by it. She *does* get giant tomatoes but usually only three or four to a plant whereas she should be getting 20-30 medium-sized tomatoes over a period of time. She swears these gigantic tomatoes are better but they have less taste than the ones in the store, in my opinion (and don't get me started on those darn topsy turvy planter thingies... UGH!).
I don't think there's any harm done and I'm positive the vegetables will be fine for eating (which was my first concern) but I wouldn't recommend continuing to use it.