I volunteered with BB/BS for 2.5 years, all with the same Little Brother, until moving out-of-state last week. We became friends, I think it was a great experience for both of us and I highly, highly recommend it. There is a serious shortage of male volunteers and you can make a big difference with very little time or effort.
Initially, prospective Big Brothers fill out an application, provide character references, and submit to a background check and, later, an interview. Interview questions included my interests and preferences about a Little's interests and background (including preferred age range, ethnic background, etc.). Then I was shown files of 3-4 potential Little Brothers and asked to choose one. After making an almost arbitrary choice between what looked like very similar boys, I met the boy and his mother in the presence of my case manager. We immediately went out for ice cream and got to know each other better. After that, I typically picked up the local free paper when it came out on Wednesday, scanned the "Kids" section of things to do for the upcoming weekend and found several events that looked fun for both of us (soda pop tasting, museum exhibit, air show, movie, etc.). I called my Little Brother on Thursday to give him a choice of the 2 activities and then picked him up on the weekend for a 2-4 hour get-together. If I couldn't find activities, back-ups included swimming (in the Summer), roller blading, shooting hoops, laser tag, etc. I budgeted no more than $25 a week and it usually cost much less. Time commitment is 2-4 hours a week, plustime to complete occasional 5-6 question questionnaires from a case manager (IIRC once a month for the first year, then every six months), usually done by email.
My match was very easy for me, since his mother really had her act together. She always kept to her time commitments, always had him fed, etc. Several times, I mentioned that I wasn't sure I was making much of an impact, but she pointed to his older brother, whose Big Brother had moved without maintaining contact, and he was having behavioral problems and problems in school.
The initial commitment is for one year, but BB/BS always hopes the match will last long-term, until the Little turns 18. Again I highly recommend it, as I think I got more out of the experience than my Little Brother did. I have some great memories, such as the time we did science experiments in my back yard and set the grass on fire (I had to convince his mom that the fire extinguisher training was actually planned) or the time he broke through the ice at a local pond and I had to fish him out and hustle him to my home for a hot shower and some dry clothes. I got to borrow him for a couple hours, then turn him back to his mom for the hard parenting work.
Although it may take me awhile to respond since I'm very busy at a new job, PM me if you want to ask any questions off-line.