Authorize.net has been on the receiving end of many harshly critical reviews, including reports that their practices "border on fraud," that they don't accept responsibility for their own actions, and that they treat their merchants "like criminals" when trying to access their money. Additionally, the Better Business Bureau gives Authorize.net an unsatisfactory rating, indicating that they have left some customer complaints unresolved.
However, there are also quite a few positive reviews of this service to be found. For instance, this page includes comments on how great Authorize.net is for techno savvy users and others who are looking for good customer support.
As always, finding reliable merchant services can be a challenge, but it's important to look at the whole picture before making assumptions about a potential service provider.
Going on the experiences of my colleagues as well as the positive reviews I've read, I have to give Authorize.net: