I am following this thread in the osgeo list: Can I do the same GIS tasks with OS (as with ESRI)?
Paul Ramsey states and I would not find better words:
"I think it's contingent on us as evangelizers to not over-sell. I would not recommend QGIS or any other open source desktop to someone whose prior experience was Arc* until I had a clear understanding of the use case."
The truth is that too often we tend to oversell (I guess because we have too much passion for what we do), giving people the right reason to complain about open source software.