What has been your experience? Or observation? What ever method, how successful has it been?
I did have the pleasure of working also in a volunteer program organization. Unfortunately there was little if any leadership support and after three years (with some significant dollar savings) the program was dropped.
I have seen/experienced this done well in a highly process-driven service organization (I have since left that organization), although, further to James's point, leadership changes did lead to the program faltering from what friends who are still there tell me.
I started my Six Sigma journey at an organization that had it baked into the culture. We had a full-time in-house Master Black Belt who was also a member of the Executive Committee, so Six Sigma was incorporated in the organization's vision, mission, and strategy. The goal was to have everyone in the organization (at all levels, up to and including the VP and General Manager) certified at at least the Green Belt level. In order to obtain certification, everyone had to go through the training and be an active team member on a Six Sigma project (the training was delivered online and was completed in lockstep with the project phase, e.g. if the team will be moving to Analyze, this portion of the training had to be completed beforehand). The team then presented project results to the Executive Committee, where the newly-minted Green Belt would receive his/her certificate and a little green flag with one gold star (you got a new star for each subsequent project). Shortly before I left, the MBB even coached a star performer through a Black Belt project (she was sent off-site for the training, but she received so much support from the MBB and the rest of the leadership team, and was promoted upon completion of her project and earning her BB). This effort did not feel overall like a "check-a-box, do it so we can say we did it" type of thing, although I do think the drive to have everyone in the organization get certified did lead to some projects that may not necessarily have been the best candidates for the DMAIC methodology. What I found nice about this approach (besides giving me first-hand experience in executing the methodology effectively in a service environment) is that, even though the company did not deploy Hoshin Strategic Planning formally, leadership was focused on ensuring that we all understood how and why our particular projects were important to the organization's success.
With such success, why did the program falter? The organization was sold to a Private Equity company.