While I like Massive I rarely use it and I am not too excited by the update.
 
As for Kontakt 6 the tipping point will be when 3rd Party library developers start using the new features of Kontakt 6 and someone puts out a killer library using the wavetable or something. 
 
Three new reverbs seem a little underwhelming given the options most of us have outside of Kontakt. Hosting 3rd Party effects in Kontakt would have been an instant purchase feature, as would direct integration with the S Series keyboards without using the Komplete Kontrol wrapper.
 
 

If it works anything like Reaktor 6, you will be able to run Player libraries in Kontakt 6 Player even if you haven't upgraded to the full Kontakt 6 yet (and keep your full Kontakt 5).  So the pressure to upgrade won't be as bad, since most of the ground breaking library releases that will require Kontakt 6 will (most likely) also be player libraries.  Should be able to keep you from being completely locked out of new libraries until the first sale.
 
Honestly Kontakt 6 seems kind of underwhelming, even from a developer standpoint.  I already punted on the full Orchestra when it last went on sale (I got the full strings a while back, but I've not been very happy with them).  If it wasn't for Kontakt 6 I'd say this is one of the Ultimate upgrades I'd skip.

I agree with Glyn, Kontakt 6 won't be a "must have" until it is required by a new library you want. The downside, of course, is that many library developers must lock their releases (even updates to existing libraries) to the current version of Kontakt. It is part of the licensing mechanism that NI uses. This is only for player-compatible or NKS-ready libraries though.
 
I bought K11U just last November and only installed it a few months ago. I probably will skip this one until I either need K6 or K13U.
 
Dan

From what I can see Kontakt 6 is more a continuation of the incremental updates that have happened in 5 rather than a jump forward. A bit like Waves 10.

If I did not have it already the Rickenbacker bass would be the most tempting of the new content. Not sure about the rest.

The last Kontakt upgrade I jumped on immediately was from 2 to 3. I learned my lesson after that. Every revision since has been purchased with reluctance, and each time I felt coerced into it. I will resist this one for as long as possible, but they'll get me in the end. They always do.
 
Next year NI will offer a $99 deal to sweep up us laggards. By then some must-have registered library from the likes of Spitfire will have come along to justify that upgrade.

Glyn Barnes
While I like Massive I rarely use it and I am not too excited by the update.
 
As for Kontakt 6 the tipping point will be when 3rd Party library developers start using the new features of Kontakt 6 and someone puts out a killer library using the wavetable or something. 
 
Three new reverbs seem a little underwhelming given the options most of us have outside of Kontakt. Hosting 3rd Party effects in Kontakt would have been an instant purchase feature, as would direct integration with the S Series keyboards without using the Komplete Kontrol wrapper.
 
 

Glyn is right on, on this. Especially on the reverbs.

I doubt they will do VST hosting for many reasons.
 
I was surprised that they didn't include the mod pack effects in K6.  Since they seem to be moving towards synthesis with the new wavetable feature, it would make sense to include modes from their new CHORAL effect that emulate a Roland chorus and ARP ensemble.

I have K10U. Guess will wait till next summer to upgrade when there's a sale. Always skip one upgrade and wait for the next sale to get the best value for your money.

Looks like Komplete 11 is off of the NI site now.  You have to purchase it or gets upgrades from a 3rd party vendor.

I went with full boat Ultimate Collectors Edition. 600 bones (upgrading from Ultimate 10) but I should be good to go until Kontakt 7 comes out.