I love my Fantastic Fans! I have 2 powered with rain sensors, front and rear to provide good air circulation. And I put a manual over the galley area as a skylight...or just in case I burn the bacon. They require a 14" square cut out, so if you have rounded corners on your current vents, you will have to do some surgery. But it is very simple with a metal cutting disc on a grinder. Cuts like butter...but be careful...the grinder will try to jump ahead on you if you do not cut against the rotation of the blade.
I have heard of potential warranty issues, but have not experienced any yet.
Maxx Fans are also highly regarded.