It’s not an exaggeration to say that the series would not be complete without this bonus episode, which extends the story to give the plot a satisfying conclusion (or a true ending of sorts, in Visual Novel speak).
Unlike certain other anime that choose to go down the path of fan-service or recaps for their extra episodes, here you have new content that actually matters.
The episode takes you to a time period after the meeting between Okabe and Kurisu at the end of episode 24, where the latter has moved to the US. You’ve got a good mix of comedy at the beginning (no thanks to Okabe alias Kyoma) and the tying up of loose ends towards the back.
Steins;Gate remains one of the best visual novel adaptations to date, and I have no doubt that the upcoming movie will also be well worth the time. 😀