We have Cylinder Deactivation on our CX-5 - I believe ours was the first model year to offer it. Concerning driveability, I haven't noticed any difference - you can't tell when it activates or deactivates as there is no indicator light, nor does it seem to have any effect on the way the car 'feels' - no additional vibration, or hesitancy, or any of those other issues that were prevalent on earlier CD engines. It's basically seamless.
As far as reliability, who the heck knows - it hasn't been out for long enough to be an issue on many Mazda's.
I'm not sure I would worry about it all too much. It does actually provide a benefit to fuel economy - A 2019 GS with cylinder deactivation is rated a full L/100km better than a GX without CD, and there isn't much evidence of any downsides. There was one recall a couple of years ago that was fixed with a software update, so that issue has certainly been ironed out of any 2022+ vehicles.
If your friend likes the CX-5, definitely go that route. It's a fantastic car - as long as they're okay with a smaller backseat and hatch area than the competition.
