I have traveled extensively in SE Asia and loved the food in both Malaysia and Indonesia. It is a rare treat to find food from those countries in Eugene. As far as I know it is the only Malay/Indonesian restaurant in Oregon (unless one recently opened somewhere else).
The food was good and somewhat authentic. I tried the gado gado, spicy tempeh (not very spicy, but somewhat sweet), and the singapore noodles (slightly spicy with a yellow curry flavor). Of the three, the gado gado was my favorite and reminded me of my time spent in Bali.
Prices were very reasonable and it is possible to get a small meal for less than $10.
Service was fairly quick and friendly. You order at the counter, take a number, and they bring the food out to you.
Atmosphere was basic but tasteful. Seating was somewhat limited and I had to wait for a table since the place was packed. However, a table became available quickly and I felt well taken care of despite how busy they were.
Eugene has a fair number of Asian restaurants (Indian, Thai, Chinese, Japanese, etc), but most of them are not that great in my opinion. Kopi-O stands out in the crowd as both unique, good quality, and affordable. Overall I enjoyed this restaurant and would recommend giving it a try.