I have both an M15 and an M2 (and about 40 other ergo and weird keyboards). I bought the M15 new, circa 1997 (three of them, actually) at a liquidators for like $20 each. I still have one in original condition. When I bought it I had no idea what it was or why it was special, I just liked the split design. As I've gone through keyboards in my career, the only one I keep coming back to is the M15. It was most recently on my desk for about 1.5 years, before it got swapped out about 2 years ago for a IBM Model F AT that I did a full rebuild and restore on, and is still my daily for my work machine. If you add up all the times the M15 was my primary work keyboard, I'd guess you'd get to about six or seven years in the last 25.
While I have never used the M2 for any serious length of time, you don't have to use it long to know it does not have the build quality of the original Model M boards. The keys do have a very similar feel, especially when the board is less used and tight, but the heft and weight of the keyboard, and the lack of an extremely rigid case, comes through in the typing experience. I'd still consider it a lot better than many MX boards, but it is clearly the cheap version of the M. (But I also consider the whole M line the cheap version of the F. ...and then there are the beam springs)
The design of the M15 means it has some of these "cheap feel" issues as well... it is not as heavy and massively built as the original M boards. That said, I feel like there was an attempt to match that build and quality, the design and shape of the keyboard was just a limitation. It is hard to make a heavy keyboard when the first thing you do is split it in half and perch it on longer feet. So, while I feel the build quality is every bit as good on the M15 as on the original M, the experience isn't quite as solid... but still much better than the M2. And for what it is worth, it has stayed that way for decades (although not decades of continuous use)
The M15 was my first split keyboard, and the first keyboard I used that didn't just tilt in one direction. I really like that. The feel and shape is extremely easy to get used to, and the split space-bar that allows you to assign backspace to the left side was game changing when I first started to use it.
All that said, it does have it's limits, especially put next to modern keyboards. As much as I love my rebuilt Model F AT, I'd love it a lot less if it didn't have a modern QMK controller in it that allows me to do a lot of configurations, layers, etc. I've used a Soarer's Converter on the M15, and that helped, but I'd love it a lot more if I could run a more robust, modern firmware on it. That might be a solvable problem with something like a tinkerBoy PS/2 converter, but I can't get my hands on one.
I've tried a number of other splits, trying to find something that could replace the M15. I've used the KeyboardIO Model 01, The ErgoDox EZ, the and Kinesis Advantage 350. The biggest issue is that those are all ortho, and I wasn't specifically looking for that. It didn't work out for me. I also tried the Kinesis Freestyle 2, but the switches are total garbage, and the two halves are different widths, making them very difficult to tent.
Wanting just a simple split standard with good switches and firmware, I currently have a Keychron Q11 on my personal machine as my latest attempt to replace the M15, but it isn't working out as well as I want. That's no fault of the Keychron, but I can't find a switch I'm happy with (the switches I like on most other keyboards aren't vibing with the Q11). Nothing comes close to the snap and feel of a buckling spring. Heck, for a short time I was even considering the split standard made by Brand New Model F Keyboards (the "F15"), but the fact both sides are (as best I understand) independent keyboards means doing cross keyboard layers and such isn't really a thing, and that's half the point of having a keyboard like that.
All in all, I can't say it will live up to expectations, but I think it is fair to say the typing experience, even before you get to the split/tent aspects, is better than the M2, and nearly on-par with an original M. The fact you can split it, just adds that much. Even taking away the buckling spring keys, the basic design of the split aspects of the keyboard is one of the better ones I've used. Put it all together, and if you're looking for a split board, it is hard to beat-- especially if you can find the right adapter for it and bring the configuration and ability toe layer up to date. I can't say if that's worth your money, but it is one of my favorite keyboards, and one I do keep coming back to.
Disclaimer: For the context of this post, I am only talking about keyboards for touch-typing, not for games. I've done rebuilds of Model M, Model F, and Beamspring keyboards, but never the M15. The fact I got mine new means I know they're clean, even if I haven't been inside of them. If you get a used one, I'd have no idea how much luck you might have trying to deep clean or refurb it.
