We have done sites based off of web services for GP 9. Don't worry about IIS, there isn't much you have to worry about with that. The install process is very easy. It did not require me to do any configuration with IIS. Some of the configuration is a little tricky at first, like adding users, and setting up policies. But once you get the hang of it, its no problem. We use web services for almost everything now, they were so easy to develop with. I wish there were a few more, to update and access more information which I hope is coming but otherwise easy.
We have used it on ecommerce sites and windows applications. We did not choose econnect because of the ease of the web services, econnect I beleive has more options and we did use it to create and update sales invoices, but changed it to web services. If you have any specific questions please feel free to let me know. Thanks!