Ok.... Let me share my experiences, and what I did to get almost everything that I needed, working on Android M. Not sure if experiences have been shared before, but if this can help others, I will be more than happy.
First - truly worth a shot. Just because it is M. And that most of the basic things we need in a phone, work superbly well. It's not that things are unstable at all. I have found it perfectly stable, just that some of the good to haves are not yet in it.
1) Total clean flash (people knowing me would know that I shift between 5.1.1 ROMs too, in that manner, so for 6,that was a given)
2) First installed SuperSU 2.52. Then the rom. Then the gapps.
3) I tried the Gapps mm fix (and not the one that u shared @@Me1256, coz i wasn't sure if I was doing it correctly

)
For gapps mm fix, I was told by@@mmrbest, a Club India friend, to simply ensure that I unmount system before flashing it. And on checking, it was already unmounted, so no worries! It worked well.
4) It took lots of time, as it does for all roms, for the first boot (u might feel, or panic that its stuck, but no it boots up properly. Patience needed)
5) Initially when ADB didn't work, I thought of going to twrp to transfer my backed up data to phone. But twrp was lost. And somehow, the tutorial to flash twrp wasn't working. So a workaround.....
Without flashing, I booted the phone to twrp from PC..... The hands off approach that's discussed in all such threads. And twrp was there again. Then transferred back the image onto phone, and hard flashed twrp using twrp

So that was done.
6) After booting up, while the phone was connected to PC, I saw that connection was in "Charging mode". That's when it struck me that they might have restored the old KK or JB days, where u can select the mode to connect to PC. Basically, even the file transfer mode via ADB was always there.
7) Power menu just has the power off option. So I have got an app from play store, which gives me reboot etc options.
8) Not tested all apps, but atleast these have got installed without any issues.
9) Tested whatsapp, Gmail, Facebook, Oneplus Forum app..... These work fine. Color OS camera is working too, but I haven't tested it properly. Same for ad block plus.... Its running for sure. Need to check it's efficiency.
10) Battery drain initially looked bad, since it lost 5% in 4 hours. But then I saw that I hadn't turned on battery optimization. Will report how it behaved after that. Yet to see.
11) Of course, calling, 2G data, etc a working well for me.
So all in all, I think I can certainly stay on this for a week.
Forgot to add that, apart from 1 instance of Google Play Services crashing (while I was setting up my account first time.... Which didn't impact either), there are no other crashes too, as of now.
Yeah, kernel aduitor gets unresponsive sometimes. That happened to me when I tried changing lcd density. So I edited it directly by opening build.prop.
Yeah, there's no recent apps switcher too, as of now. That's the only nagging part I would say.
All in all, as I said, worth a shot. Stable, with a few good to haves missing.
PS: Someone please help me put a hyperlink in my signature, for the first page of this thread.... Its just a link right now. I want the thread name to appear there as a link.
Edit 1 - WiFi does seem to drain battery a lot. 1 hour SOT, and battery around 81%. Hopefully it will improve in newer builds.
Edit 2 - Battery percentage can be enabled by activating system ui tweaks. For that, pull down the quick settings, long press on settings, and it will be activated. Found it on an article somewhere. Some more thoughts : Guys..... Layers Manager works well. Using it.
And trying to optimize battery using DS Battery saver & greenify. Plus there was some wifi scanning in location settings, which I have turned off. Lastly, I have reset wifi settings, by removing it through backup & reset.
All these, after doing some R&D on XDA threads. Consumption is much better after all this..... Don't know what worked, but it did.
And I forgot to mention that Nova Launcher is working well too.