Pros of boarding:
1. You can go out of town for a few days without having to find someone to care for your horse.
2. The people running the boarding facility *should* be knowledgeable and ready to answer questions.
3. You benefit from indoor/outdoor arenas that you don't have to keep after.
4. Depending on the other boarders, you can make connections and friends, and maybe even find someone to ride with.
The main con is that you really don't know what's happening with your horse when you're not there. I had a BO who thought she was being real clever by not feeding my horse behind my back — too bad for her my horse told me about it. There was another instance where my horse was being aggressively bullied by his paddock mates, and the BO flat out refused to move him. Essentially, you're in the hands of people you usually don't really know, and you don't have much control on what happens to your horse.