I've read many stories that it can be not only too expensive, but even unsafe. I'm not sure I'm willing to take the risk, also for my big money. But perhaps you have a positive experience with getting such services?

I very rarely travel anywhere that it's specifically an airport. Usually if I travel, it's by train. So the transfer does not interest me.

I guess there are both careless and not trustworthy transfer services and reliable and proven options. AtoB I can definitely attribute to the second, as my Bilbao airport to San Sebastian trip was comfortable and I can even say that it was not too expensive. I advise you to at least visit AtoB Transfer's website to get more details about this service.