We did it in June and loved it. It helped that we had never stayed long enough at Epcot for Illuminations -- don't know how that happened, but we tend to save our late nights for the MK.
The Illuminations party has a bit of an international flair, which is nice. However, fear not, these are not challenging desserts. Just good stuff, with sparkling wine. My kids loved the desserts. Even the picky ones aren't picky about dessert!
I did not enjoy standing, but when we grabbed a spot for the show, we plopped down on the warm concrete (it was June) and were plenty comfortable.
Comparing the Illuminations and Tomorrowland Parties:
We have done the Tomorrowland party 3 times, and the two are very different. The Tomorrowland party has more variety, but the desserts are not quite as interesting. The Tomorrowland party is less roomy, but much more comfortable with seating and a roof. We have been glad of that roof 2 out of 3 times we went. The Illuminations party was on a beautiful night, and I appreciated the chance to move around under the night sky with plenty of room.
I think finding a comfortable spot for the MK fireworks is harder than finding a good spot for Illuminations. I find the the Tomorrowland party to be much more of a relief.
We are going back to Disney in February. We are going to the Tomorrowland party, but not to Illuminations. I would gladly book it if I could, but we will be with an older relative, and we don't think she'd appreciate the lack of seating at the Illuminations party.