It depends on who you get, but you also have to put in the effort to reach out to them. My mentor was great to have and very helpful when I asked questions, but it's not like busy law students would go out of their way to check up on you.
Assigned mentors for other student groups really didn't mean much, at least for me. 2L/3L friends you pick up on your own are probably the most helpful. If you join groups you're interested in you might see the same people over and over again (as people active in stuff tend to be active in...other stuff too), so that can be valuable. Not just "diversity" groups for women and Asians and stuff but whatever sounds interesting to you (ADR and mock trial actually involve *doing* things and thus can take time if you are active in them), and chapters of national organizations like ACS and Fed Soc, etc.
While I'm at it, let me do a little speech about student clubs. You will hear that becoming "1L reps" of these clubs is practically useless. Yes and no. Yes, useless, because your jobs will be determined more by your GPA& interviewing skills than anything else. But at the same time, I found being involved in these clubs (the key is a lot of involvement with one or two, not mere membership in all of them) useful for several things. First, it was something to talk about when applying to 1L summer jobs, which you will be talking about to get your 2L summer jobs. Especially if you plan on starting applying to these things before your fall grades come out, membership/active involvement with student groups help because there literally isn't anything else regarding your law school career thus far. Second, some clubs limit voting for board positions (this happens in spring) to those who were already on the board in the fall. So it can be hard to *later* choose to get involved in clubs if you weren't already on the board as a 1L rep or something else.