From what I heard the aircraft is fine, a good performer and flies really nice, but they did the fatal mistake of pairing it with a new unproven liquid cooled Continental. Think that was the problem. The TBO is very low and when it comes to overhauls it becomes expensive. As far as I know the TSIOL-550 is not shared or used with any other aircraft, so one is kind of stuck for parts as well. The same airframe later became the turboprop Extra 500 which seems to finally be going the right way.
Here's an 11 year old review:
AOPA Online: Extra 400