Slimline active car subwoofers, any good? any recommendations?

I would like to upgrade my car's audio with a small subwoofer.

I don't want to play my music to people outside the car or enter any car audio competitions, just add a little bit more depth to the bass of my existing mid range setup.

My head unit does have RCA out.

I am thinking of getting on of these:

does anyone have one and could recomend a decent brand, or any brand to avoid, or would I be better off with a more conventional subwoofer mounted under the back shelf with a separate amp?

thanks