Some people manage to maintain on it (somewhat) well. My connection around here is a functional meth user with a legitimate job, and you wouldn't know that he tweaked based on his physical appearance (although if you talk to him regularly enough you definitely can tell that there's something "off" about him often). I've known other people who can dabble in it successfully, mostly older users, people in their 30's and 40's. It's difficult though, over a long enough timeline. And I've known a number of former addicts who kicked the habit and are completely functional normal folks today...
The progression of the drug is probably different in different areas. For example, at least one Australian user on this site has told me that meth doesn't have the extremely negative stigma it has in the USA, and is seen more like a "party drug" (which it definitely isn't in the USA; here it's seen as one of the most evil drugs out there besides heroin, a reputation it only partially deserves IMO). So I imagine that it probably goes from a weekend thing to a whole week thing sometimes. In the rural USA where meth made probably its biggest mark as far as it's reputation is concerned, the grinding poverty of the poor rural USA probably contributed a lot to its spread. Most people around here dive right into it, it seems like...as soon as they start doing it, that's what they're doing. It's pretty much a problematic habit from day one.
I'm able to still stay somewhat functional on it, as I only use it 2-3 days a week (on the weekend). One thing that disturbs me a great deal about meth is that it pretty much enslaves me once I'm under its influence, though...whatever the drug compels me to do, I do. If it's around me, I'll consume it all till it's gone, no question, and usually that's all I'm doing. For someone who once took pride in an ability to sample a whole variety of addictive drugs and not get hooked to any of them, it's pretty scary. I don't sleep, I don't eat, I won't bathe, many times I won't even leave me room at all. It's completely irrational behavior and I realize that intellectually, and it's fucked up that even though I realize it, I can't change it when the drug is around me. It's probably a good indication that I shouldn't be doing the drug at all.