I dont have a nagler myself but I have used louies 16mm and compared it directly to my GSO SV 15mm. I was viewing the helix neb unfiltered at linden. the naglers FOV was larger and good to the edge, the view in the center of the FOV was a little more contrasty but I couldnt justify several hundred dollars more to buy one. the GSOs FOV was lighter in colour and just at the outer edge (about 3%) there was seagulling. both could be focused to a crisp image. so the nagler is better for sure but in my mind not hundreds of dollars better. do take into account that i am very tight with my money tho....
that said, i am considering a wide FOV 9mm or 7mm ATM
oh and the above "review" is in my f6 8" dob