What are the pros and cons of being a volunteer editor for ACM XRDS?
While I don't know ACM XRDS in particular, in general the pros and cons of service of this type as a student are:

Pros:


You will get to know the journal publication process well, which will help you with publishing your own work.
You will get to know your field more broadly, and likely pick up interesting networking connections.
You will get to affect (slightly) the scientific dialogue.
You will get a nice bullet-point on your CV.


Cons:


It might be a lot of work, and thus might detract from your efforts on classes, research, other projects, or having a life outside of research.
You might discover that you hate the work, and be faced with a decision of sticking it out or quitting.


The likelihood of the cons applying depends very strongly on the particular journal, so I would recommend emailing a couple of the current editors to ask how much work it is and how much they are enjoying it.