I'm pretty fine by having the 1994 remasters and I think they do sound very good, so they are my go-to versions.
However, I'm interested in hearing opinions on the Japan remasters using the "flat" UK tapes. Are they worth the price? Do they sound *that* much better and do they justify the extra money? Also I would lile opinions on which one are essentials and which ones it is acceptable to avoid.
I can't offer any very specific but I definitely prefer the flat transfers to the 1994 remasters. When I did a direct comparison I remember getting a greater sense of 'space' from the flat transfers. But I am no expert and it is all very subjective. The flat transfers are available to download on certain torrent sites (search 'rolling stones japan') so you could download a couple to help you decide if they are worth buying or not.
Also, worth noting that the packaging on the Japanese releases is fabulous, mimicking every aspect of the original album releases (eg Dirty Work being wrapped in red plastic).
They do sound different from the 1994 remasters. The biggest difference may be that the 1994 remasters were actually remastered (with EQ choices that some may like or not) while the Japanese flat transfers are what they are : just transfers from the original tapes.
Essential to me are Sticky Fingers and Exile On main St. On these you will really hear something different. But all are essential if you're looking for the genuine sound of the original albums. Also worth noting that Fingerprint Files on IORR has the original speed restored (slightly slower actually).