Sure us real world consumers havent had the Atrix in our hands, but many people that are very knowledgeable about cell phones an how they work have! An looking at the numerous test these individauls have done an published show a real issue that this phone is not moving data the way it was intended. In unbaised side by side testing against many times inferior products.
My admittedly amatuer-ish take on this is thus:
Firstly when tested against the iPhone 4 the Atrix will be slower because, primarily, of the OS itself. iOS does not use real multi-tasking, which means that background processes (and/or open apps) are not eating into the processor use and bandwidth being sucked down (both play a part in the speed, especially webpage rendering) because they are not running, they are "on hold" until accessed again. On Android, apps and processes can run fully in the background even while the browser or test application is the main one open, increasing the load on the processor and bandwidth even during the test itself. Also, the iPhone 4 has re-written drivers (from previous iterations) to enable HSUPA (which AT&T does not seem to be allowing on any of its branded phones, including the Atrix) and decrease latency, both of which improve the speed results. As to other phones, I haven't seen comparisons to other phones so I don't know. I do know that most reviewers were "disappointed" with the 3G speeds on the Inspire too, which leads to the second point.
Secondly, the HSPA+ rollout is physically "complete" but is not in use yet, as the backhaul has to be hooked into AT&T's network and turned on. The "H" is displayed because the device's radio is receiving the HSPA+ signal, but without the backhaul and infrastructure in place, it is only passing 3G-like speeds to the radio. Remember that HSPA+ is really a type of 3G, it is 3G with additional bandwidth available...
Admittedly I might not be correct, but when I encountered those spots in the reviews at first I was worried, but now these two points are realizations I have had. Frankly, I know AT&T will not top T-mo "4G" speeds (They can't as they have more load on their network), but I know that it is coming and it is an improvement over what is in place already, and an improvement over Verizon's and Sprint's 3G speeds- potentially even at least on par with their 4G speeds too.