Although it responds well to throttle most of the time, it still has a tendency to load-up if idled for more than 20-30 seconds or held at a mid-range throttle setting for too long. No amount of tuning would completely eliminate this behavior, although switching to a hotter OS-F plug from the OS#8 normally used did help a lot.
This sometimes stuttering mid-range/transition means that I wouldn't recommend the ASP52 for 3D flying, it's more of a "pedal to the metal" engine that likes to be given its head with slippery airframe and a small prop.
Update (17 July 2010): I have tried the new OS#7 plug in this engine and it makes a world of difference. Although still not as smooth as an OS or Thunder-Tiger, the ASP52 with a #7 plug has a much improved mid-range and transition. It looks as if OS's claims in respect to this plug are true.