I've been to EP three times, one of which was in December during the Christmas event. It was by far my favorite trip and I'll be going back for Christmas every year. The atmosphere, the Glühwein, the seasonal shows, the lights, the Ferris Wheel, it all really made the whole experience much more magical than during summer. We went during the week in the first week of December and I don't believe we waited longer than 10 minutes for anything over the course of 3 days.
The weather can be an issue, on our first of two park days it was too cold for Blue Fire and I think Wodan to run earlier in the day, though they did open them towards the end of the day, and they were open the entire time on our second day. Beyond that, Atlantica Super Splash and Fjord Rafting are closed for obvious reasons, but the rest of the rides are operating as normal. I actually kind of enjoyed being all bundled up in my winter outfit, it made going into the cozy Erdinger Hütte for a beer that much more enjoyable.
And Rulantica especially shines in winter. Going from wearing full winter gear to being warm and cozy at a float-up bar in your swimming outfit is a wild feeling, and since the sun goes down earlier you get much more time to enjoy the waterpark at night, which is truly beautiful.
So, yes, Christmas at EP is a joy. I can highly recommend it.