Miyata were arguably the best of the mass production, Japanese manufacturers. The bicycles were very well designed and manufactured, and were excellent value at all price points. Unless it has been abused or just worn out, they are hard to fault.
The fact that Miyata operated their own tubing mill illustrates their extreme commitment to producing the best bicycle possible. It allowed them to maintain tigther control on the quality of the tubing and more importantly, it allowed the designers to make small variations to the to the tube's specifications in order to achieve the exact ride characteristics they envisioned. For this reason, many knowledgeable cyclists believe Miyata tubesets are superior to the standard tubests offered by other companies, where the bicycle designer was forced to design around a standard tubeset, unless his company was willing to pay for a custom drawn tubing.
Miyata was also the first of the Japanese manufacturers to sponsor a European based professional team, in order to test their designs in the toughest environment possible. As a result, Miyata tend to be incredibly rugged and reliable bicycles, though they are often slightly heavier than the competition.
Congratulations on your new acquisition. Enjoy.