It took me at least an entire day of sulking in my room because I finished Steins;Gate and didn't know what to do with my life anymore, before I realised there exists an OVA episode. Of course I watched it immediately once I learned about its existence.
In this episode, Okabe Rintarou and the others decide to go to America to support Feyris-chan in like a contest of some sort, I think. There they meet up with Makise Kurisu, and the rest is just dialog between the characters.
My initial thoughts were that I liked it. I laughed through the whole episode, especially since Okabe's character was much amplified. There was nothing serious, just a relaxing, enjoyable 20 minutes after the rough 24 episodes of pure agony.
This opportunity was also used to explain and elaborate what happened after the last episode of Steins;Gate, with regards to handling the time travel events. It turns out Okabe told the various characters about the time travelling, and they even remember parts of what happened in different world lines. We also get to see a happy ending for our (or is it just mine?) favourite OTP Okarin and Kurisu. Well, sort of.
In my opinion, this episode was much enjoyable, but not really necessary. I thought the way they ended the original anime was just fine. I mean, it was bittersweet not to see my favourite couple not ending up together, but that is kind of in the cards. I felt it was already obvious they were going to get together in the end (or in this case, after it). And it's funny, we even get to see Daru's future wife.
So in reality, I have some mixed feelings about this episode. Although I found it sweet, it also ruined my own vision of what happened after the anime. 7/10 definitely recommended.