Just created custom WCF service in SharePoint 2010.
Incredibly straightforward which is just brilliant.
1. Followed this article on msdn
2. I also needed to add the following attributes to the service class
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
From here on it works and can be referenced (“Add Service reference” in Visual Studio) from a client application.
Just remember to add mex to the end of the url to find the end point
Without the “mex” you will get a HTTP: 400 internal error.