Well, I have experience with some of them.
I had my first reservation with Kiwi but haven't travel yet.
Ryanair, easyjet and other european low budget companies could be tricky with their prices. Seems to be cheap and it is till you want anything extra or do not harm their rules. If your baggage is just a bit overweight or you want a bit extra on your ticket (extra leg space, fast track, etc.) it could double the base price. But if you are careful then you are just fine.
Others I have only used for searching.