I am a new M8 shooter but have read a lot about it over the years since the M8 was released. I have also been doing a lot of trial shooting with the filter on and off - daylight, low light, indoors, artificial light and a variety of colours as well as B&W.
Firstly if you are shooting in camera B&W images - not using the filter has a very good effect - the contrast characteristics are very good - seems you can get "blacker blacks".
The issue was with clour imaging and images with significant blacks in the frame (and I think more so with synthetic fibres).
Secondly, all the colour test images I have shot without the IR Cut filter have not shown me any bad issues. BUT that is not really an exhaustive test - just a test for my own purpose.
This purpose was because I shoot a lot with my Summicron-M 35 ASPH lens. This lens is seriously subject to filter induced flare!! If you don't understand its sensitivity to inducing flare, be careful! I had this experience on my M7 with a B&W UV filter. I now NEVER use a filter on that lens unless the climate/environment will put my lens at high risk - the little hood is very good everyday protection anyway.
So, with the M8 I saw I was inducing flare when I had the IR Cut filter on the 35mm lens - even possibly more so, for some odd reason (remembering my tests are not scientific at all - just user "preference tests").
Therefore it seems the main issue is with numerous/significant blacks.
More broadly, I think users' experiences with the M8 and the IR Cut filter vary quite a bit - there are so many variables in an image - just as mine have. These quoted broader colour issues unless the IR filter is used - colour "tones" across the image as well as some colour shifts.
BUT, certainly the "main" reason for the filter was to do with images where the scene had a number of black elements such as black synthetics against each other. The lack of an IR filter on the sensor caused some black areas to go "purple" and others to get purple fringing. In less extreme examples the blacks or part of the blacks were not truly black.
So certainly if I am shooting such a situation I'll have the IR filter on. Robert's link to the other forum image and posting highlights this issue.
But, I have been shooting some architecture with it and the building had a variety of large areas of black in different textures on the buildings -without the IR filter I saw no sign of a problem.
I also understand that in certain light situations some other colour issues arise when the IR filter is not used - maybe some colour shifting. BUT I have never read any report that says "always xyz will happen if you don't have the IR Cut filter on", except the situation where there are significant blacks involved.
BUT I have decided anyway that unless I am shooting B&W purposefully and the lighting gives added risk of flare due to a filter being attached, I will leave the IR Cut filter on to be sure my colours are fine.
With my 50mm and 90mm Leica lenses I have no issue against having the filter on all the time as a precaution since there are not the same flare risks - just as I shoot with my M7 and UV filters.
So again, putting aside the obvious main issue of numerous/significant blacks like that sample shows, I think users' experiences with the M8 and the IR Cut filter vary quite a bit - there are so many variables in an image - just as mine have.
I hope this helps.
