Without pictures, it's going to be really hard to ID. Based on your description, I wonder if its some species of Ironweed (Vernonia). Check out the link below to Google Images of this genus.

If you like it, keep it, who cares whether it's considered a weed. BUT, just keep in mind that it could potentially be something that spreads. I've intentionally planted a number of things over the years, like sweet annie, peppergrass, pokeweed, that I've later regretted because I end up fight their millions of seedlings.

Here is a link that might be useful: Ironweed images on Google

I thought I knew exactly what you meant, because I have something similar and I've always pulled it as a weed, but your description of the color makes me hesitate. The blooms on mine are usually more of a white or pale pink. Hmmm.

I definitely was not thinking along the lines of ironweed. Wish I knew the name of this thing so I could google a picture for you.

Dee

Ah! Is it fleabane?

Dee

P.S. I don't think that every photo in the link below is fleabane, but if you've ever done an image search you know not to trust every photo! But this might give you an idea for further research.

Here is a link that might be useful: fleabane (and other) images

Thank you all. Denninmi....I think you won the prize. I clicked the photos and that's definitely it. Now what?

Is ironweed worth keeping or should I pull it? The color really is beautiful,

I think ironweed is garden worthy. It's not a heavy reseeder for me but if you want to be sure, just deadhead it after it blooms.
I'm almost certain, if you want it to be shorter next year, you could cut it back and it would still bloom.