The best way to ensure that you can get value out of the tool is to put it against your own data. Sample data is great for learning the tool but true value for you will be exposed when you attempt your day-to-day data tasks in Tableau.
In my opinion, a good POC involves starting from the ground up; do not start by trying to recreate a dashboard or report you have in another tool. Start by asking a new question and exploring your data in a new way. Replication, especially when it comes to formatting issues, can be frustrating for new users as you may not know the full extent of Tableau's features.
If you build something new that proves valuable for you, continue to expand on it. Once you feel you have a good grasp on the tool, start to create some of your existing reports in Tableau. If some reports are faster to make in Tableau, that is great. If some are difficult to make, don't give up on Tableau. You may find that Tableau will be a fantastic compliment to your existing reporting structure; you may even find that Tableau can replace your existing reporting structure.
The best advice I can give: have fun. Nothing is worse than doing a POC by just checking off functionality boxes on a 3 page list.
... do not start by trying to recreate a dashboard or report you have in another tool.
This one nugget of wisdom from Dan will make your Tableau life infinitely more enjoyable, productive.