This does exactly what it says it does. Whether it's what you want is highly subjective. It yellows everything, but you get used to it. For my ST120 its a good filter.

Ok, let me begin by stating that I do not as a rule do reviews. I am of the opinion that every viewing session is purely 'subjective'. What I claim I can see through my eyes is not necessarily what you can see through yours and vice versa. This in itself almost totally negates any meaningful attempt at a review.
However, on this occasion, I felt prompted to comment on the Baader Fringe Killer because I bought it from FLO (brilliant staff), at the same time as buying the Skywatcher EVO 120mm Achromat. So my review is essentially two fold!
All those astronomy forums tell you unequivocally that unless you buy some sort of 'minus violet' filter, your viewing experience will be pointless and spoiled by chromatic aberration because you own the 'poor man's refractor'. After all, they describe themselves as 'serious viewers' or 'of a professional level', NEVER a keen enthusiast, because they have the ED glass or an APO refractor. I digress.
Back to my review. Simply put, the Fringe Killer is a superb filter and used in conjunction with my EVO 120, the views are magnificent. I don't know what it is that the APO owners think they are seeing that is appreciably better than what I am seeing but let me state for anyone in doubt - DO NOT be afraid of purchasing an achromat refractor and if you do, get the fringe killer as well because you will enjoy superb views of the planets, moon and DSO's. The filter has minimal light loss and I mean minimal but Baader has published the transmission curve data for the filter anyway if you need to satisfy yourself before purchasing. It creates beautiful contrast and darkens the sky enough but not to any detriment of your viewing experience.
I can honestly say that with the Fringe Killer permanently in the diagonal of my scope, I was picking out subtle detail on Jupiter's surface that I had not previously seen through my 200P reflector. It is true to say that when coming to focus, the object in the eyepiece shows, green/mauve hues but at focus, I could see almost no CA at all. As for the moon, with any of my orthos, there was NO fringing at all- neither on its limb, in shadowlands of craters or anywhere else on the surface.
Save yourself some cash on expensive APO glass and buy a fringe killer and decent achromat instead. Or, don't listen to me, but, check out Neil English's blog on Achromats. There's someone who really knows what they are talking about. Hard glass for him every time!! Oh, by the way, I have no affiliation with FLO or Baader. I'm just someone with poor achromat glass but fabulous views!!
Phil. N. Ireland.