Definitely a noticeable-to-significant improvement in terms of render times, overall, I'd say in PP2014.

Sorry, that's about as precise as I can be!

The progressive render option, for the purposes of fast, draft previewing, e.g. of IDL lighting results is also really useful, I'd say.

I am possibly seeing much less in the way of render artifacts when using IDL too, by the way... although that may even be since the last PP2012 service release.