I've been in the bottom 15% of my class since my 1L year. I just finished my 3L year (I have one more to go since I am a part-time student), but I'm not worried about my prospects at all.
Why? Because I work hard to compensate for my low grades. I haven't bothered with the career services office, or on campus interviewing. I sent my resume and a cover letter to agencies I was interested in working for. As a result, I've had some amazing clerkships with private firms and the DA's office. I've done Directed Research projects for professors, which has bolstered my writing sample portfolio and given me awesome new contacts. To top it off, my self-marketing managed to score me a summer internship on Capitol Hill (which as far as I know, I am the first person from my school to do).
Grades are important, sure...to the extent that you don't flunk out. But if you market yourself appropriately, you can make up for your academic shortcomings.