I've utilized Thomas for my business to promote my listing on their platform, and the results were solid and have paid for themselves.
I also know first-hand others who also leverage their marketing services (website design, SEO, digital marketing, etc) and have had AMAZING results. In fact, it was their recommendations that encouraged me to jump in with ThomasNet for my own (super small) business. I don't have the marketing budget to do it right and engage Thomas for their full suite of marketing services, but if you can swing it, I'd HIGHLY highly recommend it. Their staff knows their stuff and handle everything from ebooks and blogs to video content and website redesign (which so many people need help with!).
TL: DR I've had success with them first-hand, and know others who have had stellar results.