I have just finished working on a project using K2 blackpearl 4.5 RC. We started off with beta and then moved to RC. I was very impressed with the ease of install, configuration as well as the performance and stability of the product.
We used K2 studio, inline functions, VS.NET 2008 and quite a bit of the out of the box reports functionality. Performance improvements were pretty obvious with the designers and the reports. There were couple of minor glitches but overall it was good fun working. I now look forward to the RTM upgrade of this environment.
Have you used 4.5 on a real project yet? What was your experience like?