Chris Holmes, he of GeoServer, writes:

As for your query about ArcIMS comparing to GeoServer, as server’s they’re quite comparable. Where ArcIMS wins is out of the box wizards to make new front ends. We support a wide variety of clients with open standards, and haven’t gotten much farther than some nice ‘preview’ maps and example demos, but not a click x, y, and z and get your stuff. But a new mapping client — OpenLayers — is making huge strides, so that it’s as easy to work with GeoServer, or any WMS, as it is to make a google map. So it’s pretty easy for anyone with even a bit of programming experience. Hopefully we’ll get to super easy wizards sometime soon.