nypatentlaw's points are valid. I have a BS and MS in Comp Sci and am in my 3rd year as an evening law student, with one more year to go.
If you have the financial means, go full-time. If you don't, working full-time and going to school at night is difficult, but can be done.
I have been working full-time during the day and going to classes at night over the past 3 years. All I can say is... take the decision of going to law school VERY seriously. Its an investment of time, money, life, and sanity. I currently have a very stable job with good pay, so it was a decision I took about 2 years to make. However, it can be done - I'm still in the top 25% of my class.
The main thing is discipline and self-confidence. Weekends are just like any other work day - you wake up, make the coffee, and study. You will need the discipline to keep to your study schedule. If you're in a relationship, you will need a very understanding partner. I'm fortunate enough to have a fiance that has dinner prepared for me when I get home at 9:30 at night! As to self-confidence... there are many times that you will doubt yourself and your decision - why am I doing this? I have a good job right now, maybe I should have just stayed where I'm at. You have to have the belief in yourself that you can get through.
Also, as to grades... Its true that you compete with students that may have more time than you, but if you go evening, you're generally competing with other people who maintain jobs and families outside of school as well - so competition is leveled to some degree. Grades are important, but if you decide to go into patent law and pass the patent bar, your grades may not be as important as if you were going into another field - this is what I've been told. If you think about it though, patent lawyers do have a very unique niche - science/engineering degree, law degree, and patent bar. Having these three elements separates you significantly from the pack of poli-sci, philosophy, history, etc... majors.
Good luck with your decision.