Hi Gemini,
I've adopted from China twice. There are a lot of good agencies out there and a few really bad ones. Yes, some agencies are better than others, but the important thing is that you find an agency that is going to be a good fit for you, and that might not be the same agency that's a good fit for me.
What you need to do is try to think about things that are important to you, where agencies often differ. Some examples of that include:
1. Fees and fee structure. Some require more fees upfront, which IMO, you want to avoid. In case the agency falls down on the job, it's best if the bulk of the fees aren't due until after you have your referral. Some agencies are thousands of dollars more than others for no good reason. As long as you are using a reputable agency, it is not necessarily a case of more $ being better.
2. Additional, Non-China requirements. Some agencies have heaps of additional requirements beyond what China requires, like religion, family size, demands that one parent stay home for a given length of time, income levels, etc. For me, any agency that added on additional requirements got crossed off the list, even if those requirements didn't affect me.
3. Local. Some people want a local agency. I couldn't have cared less, and ours was across the country. I actually preferred it that way.
4. Is the agency China only or does it have other programs for which you qualify? The latter was extremely important to me. I've watched international adoption long enough to know that countries sometimes suddenly close down. While China has been relatively stable, that could change overnight and you'd be up the creek. So, I restricted my search to agencies with at least one, and preferably two, other progams we qualified for. Other people want a China-only agency thinking they will be more focused.
5. What happens if you get pg while waiting for a referral? Some agencies will cancel your adoption and others don't care. Find out beforehand if that's even a remote possibility.
I recommend the Report on Intercountry Adoption from ICC.
http://www.iccadopt.org/ It's one place where you can get all the agency specs and compare them easily. Once you've narrowed down your search, check references. Better yet, ask people on APC
http://groups.yahoo.com/group/a-parents-china/ to email you if they've worked with a particular agency and are willing to discuss their experience. It's good to check the refs the agency gives you, but it's better to hear from a cross section of people not hand picked by them.
Best of luck!