I hadn’t had any experience with the new platform prior to this upgrade and I skipped over the MK4, but the 4S upgrade is a significant step up over the 3S/3S+. I wouldn’t necessarily recommend the upgrade kit — that took much longer than expected to complete (about two days) and I regret not buying a new printer instead. But, I have a 3S I plan to upgrade to 3.5 just to get the new electronics; that upgrade is far less intensive.
If you haven’t tried out a 4S you might be pleasantly surprised by how much nicer it is than the 3S+.
I went for the 3.5 upgrade as the upgrade from 3S+ to 4 was almost as much as outright buying a new 4. I'm glad I did it this way because now I'm thinking of getting the CORE One and then I'll have 2 excellent printers.
Sure, it is a better printer, but it is clear that they are going for scale, and most of what makes them better is in the software rather than in using premium hardware.
They still seem to be thinking the primary audience of 3d printers is people who tinker. It's not been that way for a long time. People just want to be able to unbox, plug it in and print. The second you add in the "oh just spend 5 hours tweaking this spaghetti mess of an MMU" you've lost them.
I think they just screwed up the design of the MMU but they never went back to the drawing board.
But for 3D printers that worked out of the box under $1000, Prusa had no real competition itself.
The Mk3 came out in 2017 and I swear Prusa just sat on their laurels. I was a Mk3s+ owner (well, still am) and was pretty disappointed how little improved with the Mk4.
Bambu’s competition was Prusa and they clearly strived to improve over what Prusa had accomplished.