We did E night on Wed. June 16. It was fantastic, a magical night we will always remember!
At the last minute the park added an extra hour AND a second Spectro. My DH was skeptical when I laid down the extra money for the E-nt tickets that it would be worth it, but he ended up loving it.
We were afraid our youngest (5) wouldn't be able to handle being in the park until 2am but he was like the Energizer bunny, he just kept going and going! We did not have any problems with long lines. In fact, everything was a walk on. I think in Summer, the cooler evening hours are the way to go.
The next day we slept in a little and went to Blizzard Beach where you can relax quite a bit floating around. It worked for us!
By the way, we only had the one day at MK and didn't arrive there until 7pm. Thanks to E-nt we got to do everything we wanted and more!