Some are better than others. I think it could be worth it to test them out (maybe one or two at a time to see which ones actually send you decent traffic).
Like Jon said, it can clutter a site (and slow the loading times) to have 10 different "voting" buttons on a page, but at the same time, some ranking sites can give you some good traffic and sales as you move up the list.
They probably aren't a good fit for every t-shirt site, but they can be a good fit for most.
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