Dick,
Unless you have a 0.85 finder, in which case the difference is remarkable. The overall finder is not that much different, although the lines are a bit brighter and have more contrast in the MP. BUT, whiteout can be severe under certain lighting conditions -certainly with the 0.85 M6 - so much so that you literally cannot focus.
I gather the 0.58 M6 type finder is pretty good and resilient to whiteout and the 0.72 better than the 0.85.
Outside and in even light, there is no issue. However, shooting in backlit conditions, with a bright light source off axis and a darker/shaded subject to be focused... Oh, boy you can be in for torture with the 0.85 M6 finder. Pick up the MP - hey presto, perfect, no whiteout, focused with ease.
I agree the M6 is a bargain, though I have noticed ebay prices soaring above dealer prices lately !?!??! I bought the m6 0.85 in flawless condition (effectively) for about half of a similar MP. One with more wear may come down to 40% or so, but a third is not happening in the current market - not like for like condition anyway.
The 999 series seems like a waste of cash to me. Aren't the numbers painted on rather than engraved? I would get a regular one as the 999s are being pushed by those who bought speculatively but I dont reckon their real value will be uch more than regular ones when the sellers realise they cannot sell a regular TTL for the same as MP!