I don't have personal experience with the NX (though I do have an AX, and its autofocus is, too put it nicely, not so good), but I did have an opportunity to handle an acquaintance's N-Digital a few years ago, which I believe should be comparable to the NX. AF speed wasn't terrible, but certainly not on par with contemporary Canons like the 10D. For kids running around it might be alright, but I wouldn't trust it in critical situations. That said, Zeiss glass is always fantastic, and for a cheap price you don't have much to lose. Worst case scenario you end up with a great camera and lens for landscapes and portraits and other still or slow subjects.