I have one. Definitely not good enough for a turtle in a 40 gal. It's not even good enough for fish in a 40 gal IME and a turtle needs much better filtration than fish do.
An aquatic turtle such as a red or yellow eared slider is going to quicly outgrow a 40 gal tank so I would recommend purchasing a filter that will also work on the new upgraded tank it will need in the near future. For my RES in a 55gal I went with the Fluval FX5. It's about $212 on Amazon right now which is an awesome deal on it. This filter is powerful enough that I know it will still be sufficient when I upgrade my turt to a 130 gal.
The Fluval 405 which is about $141 would be excellent on your 40 gal, but would not be sufficient for anything larger than 50 gals, and unless your turtle doesn't exceed 5" shell length (which isn't the case with any of the common aquatic pet turtles), this will not be large enough for a permanent home.