I used to use Conway Travel because they were the agents that my firm used and so it was free for me to get my visas through them. I found that they were really helpful but that their knowledge of currect rules was limited. When I insisted on them having a go, regardless of what they thought the rules said, they were pleasantly surprised to report that my application had indeed been successful.
The rules that agents use are generally the same as those published on the relevant Chinese Government website, but these don't always reflect reality as official memos or edicts from Beijing can temporarily change the rules (in response to naughty interference, an expected mass influx of visitors for a trade fair or 'Univerisade'(!), or a suddenly heightened risk situation, for example). The visa office staff in Wanchai will be completely familiar with the very latest twists and turns, but, as others have said, they won't actually engage you in conversation.
Asking around on a forum like this is generally a very good place to start. I haven't got any more up-to-date knowledge than others have already put forward, but I would agree that it is unlikely they'll insist on your filing your application in the U.S., since you're already in Hong Kong, though they may take into consideration how long you've been here and what your purpose is. If someone had just landed in HK yesterday and was just stopping over for a day or two on a poorly planned trip around Asia, for example, then it's possible the Chinese immigration officer will take a "tough luck" approach. If a visit to mainland China had been part of your vacation itinerary, then forward planning would not just be good common sense but quite likely an inexcusable omission.
On the other hand, if you're in Hong Kong for other legitimate reasons, even if your stay is not expected to extend beyond a few months, and a trip to the mainland has arisen as a necessity or as a natural or logical extension of your purpose that could not earlier have been foreseen, then you'll find that most of the rules are really just 'guidelines' and your visa can be processed in Hong Kong after all.
I made the mistake of using CTS, once. Don't.
CTS, i had my fingers burnt during the 08 olympic year, won't touch them with a bargepole
This myth that somehow an agent is going to do what you can't do yourself needs to be ended.
If you can't take the time to go there - fine but it is not a huge deal. I always go in the afternoon - have the form done in advance and also my photos in hand so it is an easy and quick process.
The person near the door checks your submissions and if all correct you get your ticket to a window person who takes the application for processing.
You sit down and watch the board for your number. The person looks it over and you are given a receipt to come back and do a pickup which is even faster.
When you pick up you pay the price based on what visa you get.
Everybody did that year, regardless. No official announcement but pretty much everyone were denied multiple entry visas that year despite several previous visas. All you could get were double entry visas which were no good if you were working there weekly. Those crossing the border without HKID were having their multi entry visas cancelled at the border. Crazy times.
Latest China visa information is not published on the Internet. The official website is always out of date. The closest you can get to real time information is sites such as these where people are sharing experiences.
Do your own research online, then have a chat with the numerous agents about to get an opinion. The are quite a few at in world wide house whose brains you can pick.
it was not the limitation of double visa, they con me to getting train tickets ( i live in fanling, what a waste of 1.40mins on a back and forth which was not necessary) from hung hom to guangzhou and hotel reservations on false info as they said it was compulsary, then i asked the issuing office in wan chai a few months later, their was no compulsary rule on verification of travel tickets as a visa requirement only a hotel reservation.
This is the general rule, however, as of a few months ago I was told directly by the folks at the Wanchai visa section that certain passport holders, including USA, can apply in Wanchai for visas. I posted about the experience in this thread:
http://hongkong.geoexpat.com/forum/54/thread241342.html
I think it was best explained to me as being a 'rule on paper, and a rule by practice'. On paper you are supposed to apply in your home country, but in practice they are (as of that last thread) allowing people to apply here in Wanchai. Should the US sell jets to Taiwan or a carrier battle group do exercises off North Korea or the Dali Lama visit Hollywood or any number of other 'offend-the-sensibilities-of-the-Chinese-people' type events take place, the rules in practice can change quite quickly.
If you are here already, give it a shot. If you are planning for a visit in the future, apply back in the USA and have it before you leave.
Good luck.
CTS always have the most up to date information, end of story. If they charge a few hundred dollars more, so what? If your rate is $200 dollars an hour then maybe...
You can call CTS and they will give you the information over the phone for free.