Kigali has two tiers of eating: international restaurants catering mostly to foreigners and expats, and local places serving brochettes and buffets. Street food is virtually nonexistent compared to other African capitals.
The international restaurants are some of the best in the region, but you will pay for it — expect around 10 USD per main dish. Local places are much cheaper, with lunch buffets downtown running about 3 USD. Indian restaurants tend to be the fastest option if you are short on time, and the quality is genuinely good. The city's Indian community has been here long enough to get the food right.