Depends on how you look at it, in regards to myself.
Before I started playing DnD, (around the same time that I just started going out with my girlfriend) I was lucky enough to have a fairly decent social life: I had different groups of friends that I would hang out with and go out fairly often, mainly to the local night clubs (I love to dance, heh).
Anyways, as the years passed and I grew more comfortable with my girlfriend, I started losing contact with my friends (wanting to spend more time with her; while I still have a few "friends", only a few are left that I would consider "close") and going out less frequently.
However, the only constant was playing DND, which I actually made the time and effort to keep an active part of my life, as oppossed to the above. Nowadays, I find DND night to be the "going out" night of my life, the other days and nights spent either at work, school , at home just relaxing or with my girlfriend.
When I stop to think about it, a lot of my "wandering" thoughts tend to gravitate towards DnD. I would say that from a very black and white point of view, I pretty much just do work, school, girlfriend, or DND. I'm fairly happy, so I don't think this is necessarily a bad thing, hehe.
I don't know if I would say that DND is a "lifestyle" for me, but thinking about it, I would say it is a VERY big part of my life for the past couple of years.