I used this card while in HK and the mainland a couple months ago. It worked great - definitely worth it to avoid the firewall if nothing else. I also bought China Unicom's Japan SIM to use while we were there, and didn't have any problems.

There is, unfortunately, no real way to check your usage on the HK+Mainland SIM while you're on the mainland. Thankfully our phones kept track of our data usage, and they ended up saying a little higher number than what we actually used. The initial setup was a little bit frustrating, but it only took about half an hour for two phones (25 minutes for the first, 5 minutes for the second once I figured it out).

We ordered all the cards from China Unicom Hong Kong's website, and they were delivered to our hotel the day before our arrival, as requested. All told it probably cost about $15 more than I wanted it to, but that's a small price to pay for connectivity, and not having to deal with finding a phone store, getting the right card, not having to avoid upsells, etc.