IES is a b@stard to use. I worked with some consultants who modelled the environmental performance of one of my buildings with it. There are big problems with creating the building model though. Hopefully this plugin gives them a better way of doing that.
IES is also very slow in its analysis. AFAIK it is a tool aimed at engineers rather than architects and requires entry of some quite technical data, as it takes account of the proposed heating systems and lots of HVAC stuff.
The consultants I worked with have stopped using IES and replaced it with a system called Hevacomp. Hevacomp has been bought by Bentley, and is based on the EnergyPlus analysis engine, albeit heavily modified.
I have tried the EnergyPlus SketchUp beta. It kind of works, but this sort of thing isn't easy. I don't think IES is going to be your thing if you want to do energy analysis with SU.
I would be really interested to hear anyone else's views though.
[Edit: I just noticed that there is a free application called VE-Ware on the IES site which works with both the SketchUp and Revit plugins. Something to try out next week I think...]