Unlike most prequels, Caprica not only provides a backstory but also enhances your appreciation of the original source material. There were a lot of things that bothered me about the Battlestar Galactica reboot, including the fact that the Cylons seemed so human and were monotheistic. It seemed like too far a leap for robots. Caprica provides a neat explanation for these.
First, we learn that the Cylon robots were imbued with a human personality. There's a fairly realistic (or "realistic") account of how a human personality could be transferred to a robot, basically treating the human soul as an aggregation of the data stored in the human brain. The personality transferred was also a devout monotheist which explains how that "trait" got passed down to subsequent generations of Cylons. And to boot, it's the personality of an emotionally troubled teenage girl - what a neat twist!
In addition to the Cylon origin story, Caprica also explores other moral issues. One theme throughout the series is the dangers of a virtual reality world, not unlike an amped up internet, in which people are allowed to act without inhibitions. For some, this part might seem out of place. The frequent images of teens clubbing and 1930s noir gangster scenes are a bit jarring. However, they're not just extraneous - they come to an interesting resolution near the end of the first season.
The characterization is a mixed bag. I love how Zoe Graystone, a teenage girl whose "programming" is installed into the first Cylon, struggles with so many issues, such as faith, reality, and love. The Adama family saga is also interesting. We'd heard a bit about Joseph Adama in the Battlestar Galactica series, but seeing him morally compromised provides a fascinating insight into the future Bill Adama's family.
The Graystone parents on the other hand are a mixed bag. Eric Stoltz brings a compelling mix of gravitas and weariness to Daniel Graystone. When he's depressed, you really see the weight of the world in his eyes. However, both he and his wife, Amanda Graystone, often find themselves making stupid decisions. In a few places, Amanda just goes off the deep-end in doing something incredibly stupid. It never completely destroys the show because while the decisions are stupid, they are human. However, I found myself losing respect for the characters as the show went on.
The show actually came to a pretty fitting conclusion at the end of season 1, despite having been cancelled. One other thing that bothered me though was that it seems the producers felt the show needed more action and intrigue. By season 1.5, some of the more interesting character development was dropped and the subplot with the terrorists became more prominent. By the final episode, the Graystones are running around Caprica trying to essentially stop a ticking bomb. There are plenty of other "ticking bomb" shows like 24 and I think Caprica lost a bit of something in trying to mimic them.
Overall, if you liked BSG, I think this show will help you make sense of the Cylons. Even if you weren't a BSG addict, Caprica is an interesting sci-fi show that had a lot of potential and certainly didn't deserve to be canceled.