I actually haven't heard of any players which moved from Minecraft to any game on Cube Engine 2; probably as it's a way different genre which only seems to share the similarity as being cubistic (woohoo, correct me if I'm wrong).
However, I do have heard of players which moved from any game on Cube Engine 2 to Minecraft...
Well, the way I see it, it's all good. They're all good fun games. There might be someone who has been building with minecraft for a while, and wants something with a bit more realism, and might transition to sauer or tesseract once they see a few screenshots and edit with it for a bit.
Quake Live, Open Arena, Xonotic all have editor programs and mappers, and we already have a xonotic section, even if nobody is mapping for it. They're also good and fun games.
And similarities, well, they're games, they're 3D, and you can cooperatively build. That way you get to meet people and see their skill. I think it's lots of fun. Cube engine 2 games are a bit more hardcore since you have more freedom and there is no crafting.
Personally, I'm all for it, bringing more people into the clan, building houses and whatnot in minecraft, then in tesseract, maybe even importing maps from mc to tess. The downside of tesseract is that you have to build everything by hand, and even after you build it, there is no AI to populate your world. I think a better competitor to minecraft would be a cube2/tesseract based RPG with spawning NPC's and whatnot.
I am aware that some people don't like minecraft because it's full of dumb kiddies who think they're funny and post youtube videos and whatnot, but there are interesting people as well, and that's what the purpose of the clan recruitment system is - to pick those interesting people and build with them in a more relaxed fashion, since nobody's going to grief or act immaturely. We're going to have two mc servers - a public and a private one, the private one will be for clan members only, and the public one will be my project for monetization from donations, and a way to meet people. Lots of us will have admin access, and maybe a share of the earnings. There will be a lot of players connecting, hopefully we can bring it to hundreds with proper promotion. I'm still trying to learn how to make it interesting for players, and how to administer the server hardware as well. That's why I upgrade the server recently.