I often hear people who maintain bird feeders in their yard complain if they are attracting grackles instead of the birds that are more visually interesting. Common Grackles are large enough that when they come in for food, they will usually startle any of the smaller song birds that are already there. And then there is the fact that they can eat quite a bit. If you have just a few Common Grackles making visits, it might be manageable, but if a larger flock comes through, they can literally clean your feeders out in a matter of an hour or two.
I will get Common Grackles at my feeders. I usually only see one or two at a time (I think there is a pair nesting somewhere nearby). They will spook most of the other song birds when they arrive, but they come and go enough to allow other birds their chance to feed. And if I see the grackles hanging around constantly, I will let the feeders sit for a few days without food just to get them to move on to another food source.
Despite their annoyances, Common Grackles are still interesting to look at. Their yellow eye really stands out. And while the iridescent coloring on their feathers may look oily, it does give them a touch of color. Here is a photo of a Common Grackle in my backyard.