1- Read the usage policy of the website. I know this is the third time I mention that, but that tells you its important :)
2- Always try to use the mobile website as it will generate a smaller html pages, so faster load time on your app (for example http://m.imdb.com)
3- The library is powerful, but if the structure of the website changes, then your app will crash. But you can fix that by following the hint number 4
4- Instead of using the HTML Agility Pack on the client side, you can create a server that parses the content from the website, and provides the content to the clients as JSON. In case the structure of the website changes the clients will still have the old data, and you can fix your server to adapt these changes. Also notice using this module will make your app runs faster, as JSON files would be more concise than HTML pages.

Hint when use HTTPAgilityPack