I've seen HTML agility pack mentioned quite a few times but it doesn't look like its been updated since August 2012? Has anyone used this recently and encountered any problems? Does c# have anything built in for this?
First of all HTMLAgilityPack it's not for simulating user interaction in a web page, HTMLAgilityPack is an agile HTML parser that builds a read/write DOM and supports plain XPATH or XSLT (you actually don't HAVE to understand XPATH nor XSLT to use it, don't worry...).
For user interaction, full load of the web page, web testing I strongly recommend you Selenium, Selenium automates browsers. Selenium has support for several programming languages (Java, C#, Ruby, Python, etc), you can read more in the above link with a very good documentation.
The only drawback of Selenium is its open a browser to make the work, but it can be simulated in some environments to run headless browser, you can read more about this in the following links :
I hope this help you