I've used Traders Link a couple of times and I've never had any trouble with them. Many of my friends have used them too and were happy. But there's always a question mark over these visa agencies and if you can get your visa through the official channels that's better.
I found the article below quite useful. It discusses getting visas through agencies or through your work.

Some of my friends received the visas from a law firm whose name is jay and shaw. My friends said that the firm is with years of experience in helping expats living in China to deal with legal issues. Its specialties include but not limited to prenuptial agreement, marriage, divorce, inheritance, lease, real estate investment, labor and employment, IPR, establishing business, criminal defense, civil representation, etc. In addition, it has a close networking with other business agencies visa agencies included. Its mail seems to be inquiry@jayandshaw.com.