I'm not a Carrara user, but I do find this thread interesting, as I have had recent experience with the software.  I recently encountered a situation where I needed to use Carrara, in a collaboration effort with another artist, who had been using it for many years.  He gave me a crash course in the workflow and overview of the tools.  We worked together on a project he had started in Carrara, and I used the package  extensively over the course of two months, for both modeling and rendering output.

To it's credit, I found Carrara was very easy to learn, and I found the workflow very intuitive.  I found the shader system to be a little old-school, compared to what i am used to in 3dsMax or Maya, but it's deep once you get into it, and I could see the potential of some serious procedural power there.

I didn't do much animation with the software, so I can't speak to the tools and workflow there,  and I'm somewhat glad for that, because if they were anything like the modeling tools, it would have been an ordeal.  The real issues with this package were in the modeling tools (or lack thereof).  Modeling in Carrara was a slow process at best, and I found myself firing up Max on more than one occasion just to kit-bash and export objects really fast.  Even under excellent tutelage from another artist, who seemed intimately familiar with Carrara, it became painfully clear it just didn't carry a robust modeling toolset.   If Iwere a hardcore Carrara user, I'd be fighting for a much more robust modeling system, with more edge loop control, and better selection tools.  Even the artist who used Carrara as his primary production package turned to another modeling package to do most of his polygonal building, because he freely admits there's a few shortcomings to Carrara in that regard.

Carrara's modeling system is very poor and limited.  I'm not even sure why it's there. I would say if users have any hope to get Carrara into a positive light in the 3D world, that's a good place to start improvements.  Literally, I felt as if I was using a modeling package from back in 1996.  Has it been updated at all over the years?

As for the render engine... do you have benchmarks to prove that Shade is as fast as you are suggesting?  I rarely hear of it's use in the VFX or architectural industries, where fast, reliable rendering is a must.  Just viewing one very good example video rendered with the software doesn't prove anything at all.  That's a work done entirely by one excellent artist, who probably could have done the same level of work given any above average render package.  I don't see evidence that Shade's render system is any different, let alone better, than any of the cutting-edge packages out there today.  I looked at it's specs, and it uses most of the same algorythms (radiosity/path tracing) and data structures (photon mapping) to render it's scenes as other render packages, like Vray or MentalRay.  I think unbiased GPU render systems, like Octane, are probably faster, but it's hard to benchmark a GPU engine against a CPU I would think.  Then there's the Corona render engine for 3dsmax, which is undoubtedly the fastest render engine that i'm aware of today... it's still in Alpha dev, but some of the benchmarks are unbelievable, and it looks more than promising.

I think the engine in Carrara is fine, but it simply needs to be updated with some more enhancements, and include some newer rendering technologies and algorythms, if possible.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.