I've used it religiiously for years, combined with SuperThrive. If the plants are very small (seedlings or liners), I use it at half-strength. Have never had problems with fertilizer "burn" when using it and it does seem to work.
Sometimes, instead of watering it in, I dip the plant, pot and all, in the bucket so I know the rootball is totally wet (when the pot stops bubbling). Started doing that many years ago when I realized that the peat medium used for a lot of annuals is difficult to rewet if it's the least bit dry.
Also works well with bare root plants, soaking them for an hour or so before planting, then adding more after it's planted.
All the big box stores carry it. If not Miracle Gro, there are other brands that are usually called transplanting fertilizers or starter fertilizers. Just make sure it's not for lawns.
Monica