This is a long-discussed topic. I read lots of posts but I still does not find an answer.
I am dumber than Dumbo, and I never managed to start Wayland successfully, so I ask to you:
Does anybody managed to start a Linux game (bought here on GOG.com of course) on top of Wayland/Xwayland?

That vaporware happens to be the daily used system for all but one computer I own, that one being my gaming computer that has a Nvidia card, and it works completely fine for pretty much everything those systems need to do. So yeah, it's hardly anywhere near vaporware. You are right that to play almost any game you are going to need XWayland and, admittedly, I haven't bothered trying to play most of the games I own under it because I have the gaming desktop. When I get back home tonight I'll give it a shot and see what games I can get running.

Fedora 25 will be using Wayland by default, so I would assume it's quite usable (as long as the drivers support it, which means nope for those using Nvidia's official drivers until they manage to fix their shit, and for those that are masochistic enough to insist on running Catalyst on older AMD/ATI GPU's).

Did anyone find a reason to use it right now? I retried it like 2 month ago and it's performance was lacking while offering nothing that would appeal to me (right now).

Basically Wayland is bitmap-oriented windowmanager, which is how modern hardware works. Where Xorg is command/protocol/vector-based, which how hardware worked before all the 3d-stuff (windows 3.11 times, GDI-thing).
Xorg creates windows for everything and its protocols are nearly 99,9999% of times bypassed anyway and create a lot of overhead on redraw/scroll.

The performance is lacking only when run through XWindow, because its then Xorg+Wayland instead of Xorg; where it should run Wayland only.

I bet you were running the two in bunch instead of Wayland only.
If major wrappers and toolkits switch to Wayland, then Xorg/XWayland would be unneeded and only then you get Wayland performance. One just has to recompile SDL, majority of Linux games run with system SDL anyway.

Only newer SDL supports wayland, and SDL transition is not that easy.
Most of old SDL-based games may never have a newer SDL version.

I think the most promising route is build dosbox and scummvm with latest SDL.

no, it doesn't really make that much difference for the normal user.
It's mostly interesting because it gets rid of the 30year old, archaic X11 architecture and replaces it with something simpler.
I doubt the performance wins due to getting rid of the X11 overhead are that noticeable.
It should not matter anything for gaming anyway. When running opengl with direct rendering all commands go straight to the hardware without roundtrip through the X server (afaik).

the only interesting thing for users about wayland is that you can properly sandbox GUI applications, so that not every app has access to every keyboard input.

Drawback: This makes it more difficult for a program to be able to take a screenshot or video, hence some way around the restriction will be necessary.

(On the other hand, this also makes it more difficult for malicious programs to do this.)

As a CS researcher who worked with X implementations to retrofit security features into them, I am super excited about Wayland's prospects.

As a user, hell no, no reason at all at the moment! X.Org is a mess internally, but it works.

Great! Thanks for the replies, guys! It's as expected. ;)

Other discussion on the topic:
https://bugzilla.redhat.com/show_bug.cgi?id=1289714
http://forums.fedoraforum.org/showthread.php?t=312136