Build quality and reliability are not concepts that are model-specific. The construction is the same on all of the Imagine line and the same vendor-supplied components are used. For example, Lippert makes the chassis, right now Dexter probably supplies the axles and brakes, Dometic or Norcold the refrigerator, Airxcel the furnace and possibly the AC, etc. This is true for almost every brand of RV out there. Ask other Imagine TT owners about their experiences in general - no matter the Imagine model - and you'll get useable feedback.
This said, there's not an RV out there that won't have an issue at some point (some sooner than later). It's also important to ask people about their experiences with Grand Design when they needed help (as [MENTION=25010]Rene L[/MENTION] related above) - something Grand Design is known for.
Rob