I've had my "maxed out" M3 for about a week and it is absolutely spectacular. Looks stunning, too (Silver!). The white border gets criticized, but I like it a lot better than the black like on my mid '11. Also - it has nice slick feeling metal, whereas the texture on the older models have a cheap rough texture.
The touch-ID is amazing. No more password (well...waking it from sleep, that is) and is essentially instant-on.
I'm trying to understand its RAM usage, though. It has 24G, which is drastically overkill for me which consists entirely of email and message boards, news, and so forth, though the RAM is always (almost) maxed out and even swaps are used (less than, say 1 G). I do have Preview open but that file only weighs 7 GB. So I wasted $400 unnecessarily or it is doing its job as designed (yes, I understand, for sure the latter). Either way I'm good with it and I am glad I upped the RAM because there is no downside to it. Sizewise (display), there is a huge difference coming from the 21.5", something I didn't expect. One more thing : it runs stone cold and I can't hear the fan even when I press my ear to it.