These card sleeves are much better quality and more accurate (art-wise) than the 1998 versions, plus they've done all of the studio albums this time, not just the first eight.
Sound quality is up for debate. It seems the older analog-recorded albums sound better but the digitally mastered ones (from 'The Works' forward) seem to have adjustments which reduce the midrange, and the 'Innuendo' album sounds very uneven for some reason. (Still good quality in all cases though, in my opinion. You can always make some adjustments yourself)
The 1998 remasters were done with 'No-Noise' processing and some have complained that this results in a slightly dull sound. (I agree, but they're still good)
What I would recommend to someone who has the '98 versions is to start with the later albums in the 2004 versions.. the ones you don't already have in 'Crown Jewels'. This will show you how much nicer looking the new ones are and give you a taste of the whole collection before you decide whether to replace the '98 versions.
This is the way I've been doing it. I've gotten 3 discs per month for 3 months so far, starting in reverse order, so just this month I finally got the first two which I already had in 'Crown Jewels' (Jazz & The Game) and they are so much nicer than the '98s! 'The Game' has the original limited edition metallic mirrored cover which I've always wanted.. I passed up the vinyl version of that a couple times and regretted it. And I do think the sound is clearer. 'Dragon Attack' really stood out to me as sounding so much better.
These 2004 versions have actual miniature inner sleeves just like their vinyl counterparts, and the printing is so much better that you can actually read them. They all also have giant fold-out lyric sheets. And there's posters where applicable.. Made in Heaven, and the Jazz nude poster.. though they actually attached it to the cover which seems a little odd.
I think if you buy a few of them, you'll fall in love and want the whole set.