The taxis you are terming as "illegal" may not be so. There are many cars plying the roads in Qatar which are owned by companies and they have permission to run them as taxis. They may not have any stickers on their doors. I have always preferred to use such taxis over Karwa simply because the drivers are polite and prompt and punctual and are available at your doorstep whenever you want them to come. My experiences with Karwa taxis have not been a pleasant one most of the time when I have used one. And this experience has been over several years. I always go for a private taxi whenever I have the chance to do so.