I listened to a few of the stereo remasters on Youtube, and they convinced me to buy the box set. The later albums have fairly negligible differences, I admit, but for the earlier albums (Please Please Me through Rubber Soul, and maybe for Revolver too although I haven't listened to any tracks from that yet), it's like night and day compared to the old CDs. The old CDs, up through Revolver, plus Past Masters Volume 1 and the first few tracks on Volume 2, have a thick layer of hiss and a distinctly grainy sound. Furthermore, even for the later albums, I would love to have them if only for the enhanced liner notes with original album art.
The only problem is that on the songs that were originally in mono are now in false stereo, creating a weird "echoey" effect. It's especially noticeable and distracting in "Misery", on the "dun dadun dadun dadun dun dun" piano bit, where it seems to bounce from right to left.
This could be reason to get the mono box set, hypothetically. I got the stereo box set, however, for two reasons. One, it's cheaper. I got mine for $105, including USPS Priority Mail. Second and more importantly, while I don't want stereo versions of the early one-take mono tracks, I also don't want mono versions of tracks that were created in full stereo. Luckily, I can have it both ways while still only having one box set, simply by ripping the false-stereo albums as WAVs, using Audacity
to make the stereo tracks into mono tracks (I tested it with a YouTube video of Misery and it works wonderfully, the compressed-stereo seems to sound the same as the mono, also on YouTube), and then burning the now-mono tracks to CDs, putting them in the boxes, and placing the original false-stereo CDs into jewel cases and storing them. Problem solved.
One more note regarding what sewi said. Most people who listen to the Beatles (or any other musicians) are not die-hard fanatics like us. Most people just want something to listen to while jogging or driving or whatever, and so they buy the CDs, get the uncompressed WAVs or FLACs that they can't find online, and then sell the used-once CDs again. It's still legal to buy and own these CDs, however. As long as you own the physical medium, you have the license to listen to it. So when you buy the used-once CD, the license is transferred from them to you, and thus it's not you, but THEM who are illegally pirating the music. So while it may irk the die-hard fan, it's still something you can take advantage of, because the artist still gets his money, but the resold CDs are usually much cheaper than list price.