Kind of did this on a lark, but I've been curious about JunOS for a while. Never touched a Juniper device in my life, but I'm gonna be job hunting soon, so being somewhat familiar with the OS can't be a bad thing.
Basically just studied from the pdfs, threw together a lab in GNS3 and watched the CBT Nugget videos. I think I spent more time getting Olive working in GNS3 than I did in actual lab time.

That 50% off voucher is really nice, especially since it's basically a gimme.
I would describe JNCIA as CCENT + Basic QoS - switching - IP services.
It's not a particularly hard test, although I had to guess more than I would like to. Really easy to narrow it down if you've played around with the CLI at all in most cases. Judging by my score, I got the vast majority of my guesses right. The passing score is only 66%, so it seems like they really want people to get this cert easy and cheap.