with anything requiring a reflex save.
Which is...?
Knockdown feat is good, but is str based, so if you are aiming on having a high ac using tumble and dext, your strength is going to be med. Of course there are items that can be crafted or purchased that will help with that, but personally, I prefer my 5 attacks vs having to take feats of knockdown and imp knockdown, to be able to knock a giant on his butt. Plus if you go Sd you get hips that helps you with your sneak attacks.