I ordered the stereo set before I wised up and ordered the mono set, which hasn't arrived yet. The following is partly about the stereo discs, partly about controlling expectations, and might prove helpful to anyone considering buying the stereo set, were they ever to come across this site.
The stereo mixes tend to be clearer than the 1987 editions, with better separation of instruments. It's possible to pick out more details. But I wouldn't say every 2009 remaster sounds mind-blowingly better than its 1987 counterpart. With the second disc of the White Album, much of Let It Be and Abbey Road, and the last few tracks on Past Masters II left to listen to, I'm finding that there are four categories:
1. The remaster sounds almost the same, but not as good. On the stereo set, this usually has a lot to do with the bass being less present, at least up through Rubber Soul, and even parts of Revolver and Sgt. Pepper.
2. The remaster sounds different, and not as good. Sometimes separation of instruments is bad. Sometimes the instruments teamed to create a sort of undifferentiated instrument track, so that you're hearing the bass and guitars and sometimes piano as one entity. Hearing all the instruments separated can be jarring; maybe in the future I'll get used to this as the standard, but for now, I'm not preferring it to the '87 issue. I'm finding this to be the case with some of the songs I like the most. I liked the unisound I got in '87 a lot, so change seems bad. Hey Bulldog is a great example of this, and so is the end of Dear Prudence.
3. The remaster sounds different, in an interesting way. Too early to tell if I prefer it to 1987, but the instruments I'm hearing anew make for new information, and it's exciting. I didn't know that John yawned in I'm Only Sleeping, and I didn't know that's what John's guitar was doing in I Saw Her Standing There; on the 1987 disc, it sounded like it only had one string. This tends to happen more with songs I like but don't love; I noticed it on several tracks on the White Album. My favorite example so far has been Hey Jude; it seems basically the same as the 1987 master, maybe a little warmer, but at the end, Paul's scatting is more audible, and the track fades completely out just a little later, so you hear more of it, which corrected my impression of his last scat on the 1987 version, the "I said a-na, na na na na," which I would have scored one way before but now see he's singing differently. Also in this category are the stereo mixes of albums formerly in mono, most notably For Sale, which I find interesting, but am not as enthusiastic about as I would be if the bass were more prominent.
4. The remaster sounds very different, and much better. Some of the old masters just seemed mixed poorly, or mastered poorly, or produced poorly. The two chief examples were I'm Looking Through You, where the keyboard was too trebly and harsh, and Lucy in the Sky with Diamonds, where the keyboard also seemed a little abrasive. In the latter remaster, the guitar(s) in the right channel are now louder, and the mix is perfect*; with the former, it just sounds like they maybe took some of the brightness off the keys, or the channel. Also a lot of Paul's Rickenbacker bass work in the second half of the output is now more audible, including the high end, which salvages even tracks that otherwise might not be preferable to the '87 masters. And toss in songs where voice or classical instruments are prominent -- Eleanor Rigby, Yesterday, and most notably Blackbird, which I previously thought featured a subpar vocal performance. It's transformed on the new stereo set, I feel. On first listen, there aren't too many songs like this that are just in every way better now; Piggies comes to mind as another one.
So ultimately, I agree with the reviewers with early access who posted in the weeks before the stereo set's release: If I weren't a hardcore fan, I wouldn't replace all the discography right now. I might pick out a few favorites, especially Sgt. Pepper, Magical Mystery Tour, and, I assume Abbey Road, and leave the rest until my kid lost or scratched my 1987 discs. But if you're crazy-go-nuts for the Beatles, there are enough improvements and enough interesting differences to make the purchase worthwhile. A person with much more time than I have might have a lot of fun mixing and matching between the 1987 and 2009 releases for a discography that combines the best of both.
* I keep saying 'mix,' so I want to go on record here that I know the difference between mixing and mastering.
Late update: Obviously, it was stupid to post this before I'd listened to everything. So let me add here that Cry Baby Cry remastered is a landslide winner, and that it's great to hear the detail on Come Together; if you didn't believe John was saying "Shoot me," you will now.
3 days ago