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.