While the 509 looked and felt nice at a couple of trade shows, I confess I've never fired one. That said, I was employed by one of the few agencies (four that I found) that foolishly issued the FNS-9.

The magazine release appears the same on the FNS-9 and the FN-509. If it works the same, it is easy to activate and drops the mags smoothly. It is ambidextrous and you may find that problematic depending on how you carry the pistol. We had magazines released when officers brushed against walls, center consoles, whatever. I lost one aboard the FOP bus transporting a group of us from the wall to the Capitol during Police Memorial Week.

It can be replaced by a lower profile release which makes changing magazines somewhat more challenging, but reduces the risk of "uncommanded ejections".

The FNS compact comes with the low profile magazine release. I found it impossible to eject those magazines unless I placed the weapon on a table.