This article has been updated
to show lessons learned over the past three years. The content of this article is still relevant, but is a little out-dated.
At my current job, we have several external systems that interact with Salesforce, and they do so through web-services. This document will cover what I have learned in regards to web-services, caveats with them and common pitfalls.
The goal of our web-services is to provide a single point of entry for each major object represented in Salesforce. A major object would be Account, Case, Case Comment etc. The reason this is differentiated is that for instance, Case Groups would under the AccountAPI since they are a minor object. Each web-service consists of two parts. First the actual web-service class which holds the externally facing methods and from which the WSDL is generated. The second part is that of the util class which holds all of the logic and is reusable.