I've been scolded before for bad-mouthing the Tamron whenever the topic came up, but I'll do it again. I've owned this lens, as well as the 55-250, which is an excellent bang-for-your-buck performer. I don't think it is worth it to upgrade if you already have the 55-250. If you had no telephoto lens to begin with, then it would make purchasing the Tamron slightly more worth it, but for a couple hundred more, you could get the 70-200 f/4.
Personally, I'd use the 55-250 and save for something that will be a significant upgrade, rather than a slight upgrade.