I haven't used the C7520QT, but we just installed a 7017T in a classroom. Aside from having to use hex strings and calculate checksums, integrating it wasn't so bad. The hex strings for the 7520 are different from the 7017T, which may indicate a different OEM and thus different behaviors. The 7017T will let you send commands (like polling) when the display is turned off.
In all, we were pleasantly surprised by how well the 7017T worked. Pen actions were surprisingly accurate. Slightly faster than a last-gen SMART Board. We will have to be on our guard with the newer interactive models, it seems.
