Only a couple issues with the Alien. The paint and TC. The paint job isn't of the highest quality, but if you're careful you should be fine. Mine has some minor scuffs but nothing serious.
As for the TC, some people have said that TC in SS mode was shoddy on v1.2.1 (which is what firmware will come installed), but that issue was fixed with the v1.2.2 update. Some people said they never had issues at all. I'm not a huge fan if TC so I didn't look into that one as much.
Other than that it's a really nice mod. With the right set of batteries you can get a good run time without changing cells. Last week I worked a 16 hour shift, and still had about 25% charge left when I got home from the fresh batteries I put in before work.
Oh, and there is one more issue I've noticed. But I have yet to see anyone else mention it so I think it might be specifically my mod and not all aliens. I've noticed that my alien will read my builds wrong or show a short on a perfectly good coil. Like, I'll build a .3 coil, throw it on my PICO or my RX75 and it will read very close to .3 and vape as I expect. But I put it on my alien and all of a sudden it's reading 2.4 and shorting out. Doesn't do it all the time and usually taking the tank off and putting it back on usually fixes it. And like I said, I haven't seen anyone else mention this so it must just be mine.
But, early morning rambling aside, it's a really decent mod and I will probably buy one or 2 more in the future.