About the same as being 17. haha.
Though, it's really going to vary from person to person. There is no 'one answer' to this. Your choices, your environment, etc.. will all determine what it's like for you.
There really is no drastic change from turning 17 to 18, except, perhaps, being out of High School. And you have the option of moving out, which most don't do that quickly. But as far as internally or mentally or even emotionally, not really.
For me, i was active as a leader in 2 youth groups, working, hanging out with friends, all that. But also still battling bad depression and some other issues. So it was kind of bittersweet.