Not sure which to reply to but got to jump in here. Product Lifecycle Management is a huge undertaking when incorporating technology to help with it. The fact is that PLM has always existed just as collaboration has existed. (Somehow Collaboration became a new word/tool) I agree the biggest issue in implementing PLM technology is leadership or the lack there of. Again, culture is the key issue. I have seen persons in charge of and participate in PLM implementation teams that could barley create a folder in windows much less manage huge amounts of data in various process flows. As for top down, this is needed for the funding and support. But the Top down should let the Bottom up have a major role (not the sole role) in the INITIAL implementation of PLM. That is because the Lifecycles START at the bottom. It is as simple as that. PLM implementations implemented from the TOP down often struggle (they will never tell you they failed, they are still working on it) because building a roof before the foundation simply doesn't work.
As for immediate benefit, the management of CAD data is a major part to get right, including how all the tools within the CAD might be used. (Like 3D Annotation - Had to get that in there.) Why? Because it is the legal product definition. It is the design! It is where it all starts. There are no test cases, quality control plans, tooling, ERP, etc. without the CAD data. Drawings or no drawings. (No drawings is easier to manage by the way.)
However, CAD data is the start and there are additional cross functional areas that can be included in the "foundational" implementation. This is normally called the "low hanging fruit." All said and done, the most successfull team work will come from the TOP and Bottom working together without power plays or politics being involved.