I also think it is an interesting topic I am a structural technician and I have been using Sketchup for over ten years but it is not my primary Cad package this is due I think two reasons in the structural field one is the packages that are available like Revit, Advance steel, autocad and Solidworks these engines are design for engineering based drafting and it is hard for Sketchup to compete. Secondly I think it is down to cost, in the engineering Field there is a definite desire for companies to do the bear minimum when I comes to documentation to the point at some places draftsman are regarded as a necessary evil. Although I believe this is defiantly a false economy when you take into account the RFIs from site there is still the perception of increased profit.
It is down to the presentation of the documents and this is were sketchup comes into its own especially details were you can easily build up a library details. If you look at the standard of Nick Sonders documents and Dave Zachary on the engineering side. I also do a lot of presentations on buildability issues of a project where sketchup Is a great at live presentations.
I think other things you have to take into account are the job size, BIM and at what level, learning curve of staff, saying that I have use sketchup on some massive projects were it handle very large models as good if not better than some of the aforementioned.
I should also mention on some of the larger projects I have worked on there is a perception that sketchup is only a free toy 3D package (until I put them straight)
