Yup the €9 is an absolute bargain, the bahn.de website and app has been updated to suggest these fares when you're trip planning and you can pre purchase it now ready for your trip, and the ticket just sits in your app - just make sure you choose the correct month for your trip!
As
@IDB mentioned, intercity services are excluded, so although it's cheap it can considerably lengthen your journey from some of the airports that's been mentioned (Frankfurt for example would be over double the time on non ICE services!). However for things like Baden Baden airport where you're taking local transport anyway, you can save with the monthly pass even though the standard tickets are pretty cheap in the first place at about €7 each way.
If you want to easily see what trains are valid and get an idea of journey times, you can narrow your search to exclude Intercity trains:
In the DB Navigator phone app
Just under the date and departure time in the search box, tap where it says 2nd Class. Tap options an then means of transport. Tap the switch for "Regional Transport Only" and click Done.
On Bahn.de
Click the further options button, then click types of transport. Select the tickbox for "only local transport" and continue your search.
If you're planning to travel from Strasbourg, don't forget this isn't a direct replacement for the Europass 24h as it only covers travel within Germany. However
@AstroDan and I were discussing the other day, and he reminded me the Strasbourg tram (line D) now goes to Kehl. So a possibility is to pay €1.70 for a single trip in the
CTS app to Kehl and then switch to the train and utilise your €9 ticket for the remainder of your transport in Germany, including your bus to EP.
Considering buying 2 x Europass 24h family to get you there and back from EP would cost you €29 for two people, using this monthly pass and paying for the tram to/from Kehl would cost €24.80 in total for two. A small saving, but if you're throwing in visiting a city, then you've also saved yourself a travel card or two as well as it'll also cover the buses U Bahn, trams and S Bahn in major cities.