We don't have TV, so had never heard of Firefly. When Serenity was released in theaters it sounded intriguing, so we went. We loved it! Finally, a space movie that wasn't a war or alien invasion thing! Our friends loaned us Firefly and we ended up buying it.
It was very cool to go into Serenity knowing absolutely NOTHING about any of the characters or their situation. It made the movie incredibly interesting as we tried to piece together what was going on. I thought JW handled the transition from the series very well and made a smart movie in the process, even - and perhaps especially - for non-fans.
If anybody reading hasn't seen either yet, I highly recommend doing so in that order.