Once I became a working professional (Programmer, Software Engineer, Systems Engineer, other titles) the Special Interest Groups (SIGs in ACM speak) became more relevant to me. The organization has always suffered from being more academically oriented than geared towards the working professional.
I don't subscribe to the digital library (DL) because I find the cost prohibitively expensive for what I would use it for. The monthly journal attempts to cater to all sorts (professionals, researchers, academics) and I find a few articles each month of interest.
Does membership carry any prestige? As one can read from these comments, the answer is an overwhelming no - unless you are submitting articles to be published. Making it through the peer review cycle is an achievement. SIG membership gives you access to like minded folks for discussion.
Many of the benefits are now just perception as the world-wide web has subsumed most of what they offer.
Why do I stay a member? Mostly inertia, but I still value a printed resource delivered to my postal mail address rather than only digital medium for information.