Соответствие цены и качества полета. Нормальное обслуживание на борту. Есть напитки холодные(вода, соки) и горячие(чай, кофе). Можно подкрепиться во время перелета бесплатной шоколадкой или хрустиками.
I've been flying with Fly Niki for two years and I am quite satisfied with their services in general. Except for a short delay due to poor weather conditions I haven't experienced any troubles during my trips. The crew is professional and they provide good comfort to the passengers (the even offer free magazines and newspapers but only in German language). Even if there are cheaper low cost airlines, I think that Fly Niki is a trustworthy company and their services are a good value for money.
Check the cancellation policy: Usually, if you choose the lowest fare you will not be allowed to freely modify you flight details later on (change dates or passenger names, etc). If you think that you might need to do this, you should consider buying a more flexible (but also more expensive) ticket.