I really like Rhino 5. There was a big performance increase from Rhino v3 to v4, and there is big performance increase from v4 to v5 (there is also a 64 bit version of Rhino 5). Working with lots of layers and geometry is easy (including imported meshes from other apps). As already mentioned there are a number of plugins available to tailor and enhance Rhino. Including Neon a free fully raytraced viewport plug-in for Rhino 5.0.

MoI and Rhino 5 together are awesome!