I LOVE our Dynon com unit for this reason. Not only can you get the freqs at your departure or destination, sometimes I'll decide to pick up flight following mid-flight and will grab the ATC frequency from the nearest airport. Works great! I'm just installing another Dynon com in a different plane to replace an SL40 because I love the Dynon com so much.
However, as mentioned above, note that sometimes there are multiple frequencies for a given button. Not usually a problem (ATC is often kind enough to tell you to contact them on a different freq when needed).
I've only had a problem with these buttons one time: I surprised myself when flying to a nearby local airport just 12nm away. I've been there dozens of times. With our newly installed Dynon com, I pressed the TWR button and contacted tower... Turns out that airport has two tower frequencies in the database, but it's the SECOND one in the database that they use (and only the second one is listed on the sectional!). So in this case you have to press "TWR" twice to get to the proper frequency. I haven't run into this anywhere else.
Dynon, if you're reading this, can we request database changes like this? KHWD only uses 120.2 for tower but that's the second freq in your database; ideally that freq would be listed first!