Well, whether "great" or "extensive", at the end of the day it's all subjective. Yamaha may call it "extensive", but the exact list of improvements remains to be seen. My experience of life is, that you should be sceptical towards announcements from marketing departments.
Again, when it comes to valueing the forthcomping update, it's all subjective. Of course we have to wait 'til June (at least) to have the facts straight. But, what is already known today is completely useless FOR ME. Of course, even at this stage your mileage may vary. This doesn't mean I do complain, after all the update will be free. Being a loyal Yamaha customer for 20 years I'd be very surprised if the June update will contain a (IMHO, subjective!) substantial improvement. E.g. let's say creating your own user arpeggios.
Bottom line, I don't expect anything from the update, so I won't be disappointed.