Old topic, might still be useful: I can answer these questions for the A100, the predecessor to the A200.
The API is very simple to use, and provides all the hooks you'll need to interface the chassis. The update rate is flexible and can be set very high (20Hz +). The hardware interface is straightforward (velocities, odometric readings, system and power status, battery levels, etc), all published cleanly on separate topics. There are no services that I know of, but they aren't necessary for this type of hardware interface. No Complaints.
The joystick demos work out-of-the-box and will help you get started.
We interface from laptop to chassis directly via USB-RS232 interface. We've had no problems with this interface at all, other than the port-detect feature in the vanilla A100 ROS API can interfere with the gpsd_client port detect. To avoid this, set the "port" rosparam for the chassis. Their packaged launch file can be edited directly to do this. This is a problem with GPSD, not clearpath, per se.
Our package was GPS over GPSD_Client, compass and telemetry equipment via a custom board, interfaced with our own ROS nodes, Husky A100 with Clearpath_base ros node, and a custom-written EKF-based navigation routine.
See other reply for relevant links.