To respond to your questions from experience with this system:
What language is it programmed in?
Previous version - in COM
Latest version - .net
Can it be modified if their modules don't have the functionality that is needed?
Previous version - If a client had a mmc / yes | no mmc / only to a certain degree
How is Oxcyon's service?
I've never had an issue with their service. I got aggravated a few times that I couldn't do things that I wanted, but adjusted the plan for the client and got it to go through in most cases. I'm not a programmer, so I am used to adjusting to what the software allows vs. the other way around.
My issues were with how some of the forms were dealt with, and also the members integration in .com (like the last post). With the .net version there are more questions sometimes to get to where you want, but it's only becuase you have the full control of the modules that I didn't have for the client in .com.
Also, the setup of .net allows unified login, and access to whatever modules you can dream up (bulletin board / newsletter / etc.). A 'my profile' screen could have the ability to not only update their profile, but also to add news / events / classifieds / etc. to the site with approval from us which is pretty cool to have in one page. I wish we had that in .com...oh well...com is dead anyways.
For .net - It's pretty cool. You can move the module fields wherever you want, and all the functions (print / comment / share / rate / email / etc.) are done as standard scripts...so you just paste them where you want to see it. Also, they did a much needed revision to the forms management portion. You could make a form that posts to a module, or to a web based database, or any number of things. It's a little more work then in .com sometimes, but the initial template is predefined when you create a module (like news / events / etc.) so you can change it.
The coolest thing that i've seen on .net is the ability to update an xml file for the module itself, and then the fields are in the module (IE: Add relationships from one module to another, or add text fields for placement on the pate, etc.) That exists now at the client site level with or without a mmc. So they actually gave us more control in this new system.
That's just my thoughts on the system. I have been a consultant for a client using this system for a few years (5+), so if you want to know anything else...just ask. Have a good one.