No, I will not be upgrading Kontakt. I've upgraded 3 times so far, but not this time.
The library's exactly the same as K4, there is no new factory content. (Even if there was new content I wouldn't care much because the factory content is weak anyway. I use the tuba and orchestral bass drum. That's about it.) There are no new features in K5 that I care about (don't use loops).
The
only advantage to upgrading is you will then be granted the privilege of buying other NI products, which require K5. But
there are no must-have NI libraries. There is, AFAIK, not a single NI-branded sample library that doesn't have a better third-party substitute. And most 3rd-party products that require full Kontakt are compatible with K4, and a great many are K3.5+.
This situation will likely change, I'm sure. NI, like Microsoft, have their ways to make you fall into line. Ultimately, resistance is futile. But I'm not surrendering today.