Saw Garen last night and found him charming and convincing at best and hard-working at worst. Definitely not a poor performance by any means (and those who call his vocals simply adequate are doing so in comparison to Broadway's best). I have never seen Robbie Fairchild perform live, but even just watching his Tony Awards performance on YouTube, I can see what others are saying with regards to him having an upper edge on an otherwise great performer like Garen Scribner: Robbie brings a more precise, polished edge to Jerry with a certain mix of powerful yet graceful masculinity. It's hard to decribe, but it's nonetheless visible and potent. I guess I'll never know what I missed out on!
On the bright side, I was happy to catch the other leads from the original cast while they are still performing. Of special note are Leanne Cope (what a phenomenal performer), and, echoing mpkie, Brandon Uranowitz's portrayal of Adam Hochberg, which was really a standout.