There's some filler but it's mostly good filler.
Much more importantly, Mirrai Nikki: Redial gives us what the finale did not: closure. It's all very simple but handled properly and well done. Granted, the resolution still leaves you with a bit of a headache, but at least it is a real resolution, which is more than you can say for episode 26 of the show. What's weird, actually, is that I found out that the ending to this OVA is the ending to the manga. So in a sense, it's a bit strange that they created this as a separate thing rather than just adding it on to the final episode. But being someone who streamed all of this together many years later I can't complain :)
Tbh, this episode is better than Mirai Nikki itself in my opinion. I strongly recommend ANYONE who has finished the series to give it a go. It's 20 minutes that will improve your experience with the show.
