I too, am curious as to what the impressions of the EH KT90 are. I'm thankful EH began production, it is a worthwhile endeavor.
I can personally vouch for the ruggedness of the Ei KT90 tubes. I had them in my Jadis for 5 years, one shorted (no damage to the amp), and the others still measure a little better than "pass".
However, in the past week+, I retubed the Jadis with JJ E34L tubes, and the sonics are MUCH better. Of course, the power, extension, and slam is less. But, I feel the overall sound is improved. The tubes still need to burn in.
Also, I tried some burned in JJ KT88s, which seemed to restore the dynamics, slam, extension, and power of the KT90, combined with the sweetness of the EL34. It's probably the tube I'd choose.
But, if you are considering the Ei, believe me, it is one heck of a tube.