Being an owner of the modwave keyboard (v1) I was impressed. But this is actually a better build quality and takes up a lot less room. Also, although based on the same wavetables, its wave sequencing is much better - the wavestate is glorious at making tempo sync rhythms and pads. The similarities between the two are there, but the emphasis is in a slightly differnt direction compared to the modwave. It's nice to have both :)
Personally, I think the modwave is more of a workhorse. But the Wavestate is a pure inspiration machine. Now editing to get the best from it ... well it's a beast. Fortunately you can download an editor from Korg. Honestly, the number of options could be paralysing for some. However, there is that wonderful random button that will take you to places you would not have deamt of. The modwave also has this. So, you can get some crazy variations and if you hit something you like you can save it into the abundant user sound locations.
I am finding the built in fx to be fantastic. I have not tried the onboard sample funtion yet - but the capacity is very generous for importing your own waves or indeed complete sounds. I think many hours (years!) will be spent on this unit.
The quality of all the knobs and buttons seem to be upgraded and well built. Always a concern when you know you will be using them constantly.
So, in conclusion, for the money, I think this is one of the most exceptional pieces of studio gear/sound module that I have ever seen.