Here's a picture of a common grackle at our feeder. We get a lot of grackles in our yard, but usually they just feed on worms/bugs on the lawn. Once in a while we get one at the feeder. They don't bother me too much since we live in a rural area and have a nice diversity of birds at our feeders. But I understand why some people whose feeders are dominated by this "nuisance bird" dislike them.
A lot of people get grackles and starlings confused, and I admit I still have to remind myself of the difference, since I had them mixed up for so many years. In my mind, grackles should look "crackle-y" and the mottled look of starlings with their blend of black and tan feathers looks that way to me. Instead, it's the bird with the smooth black coloring (with a hint of iridescence) that is the grackle. I wrote a blog post about the difference between grackles and starlings a while back.