Have used Guided Imports as my purchasing office in China. Initially I used their A-Z service to handle a new product I designed and they helped find a capable factory and handled all the logistics, packaging, and details associated with getting it made and shipped to Amazon.
Recently I switched to their monthly plan since I have many products being made on an ongoing basis in China. I will try and update later on as I get more experience, but I'm confident recommending since I've used them very successfully previously. Handing all of my China transactions to Guided imports has made it easier to scale my business and add more products.
Overall, I've been very pleased. My only minor gripe is price, as it is with most all services. Nevertheless, I have no plans to stop using them as they have taken a significant amount of work and hassle from my schedule which has allowed me to scale.
I'll update more as I continue to shift more products to them to manage.