Pros: The price is good and they offer some of the best prices around Japan and East Asia. As far as LCC's go they aren't the worst. If you're motivated only by price, then Peach is fine.
Cons: The service of Peach is pretty much non-existent. Their staff seem to be very poorly trained, they are no proficient in English, they do no communicate well and seem to only be interested in getting you crammed into the plane (and out of it) as fast as possible. Their flights are often delayed, you can safely assume you will leave at least 30 minutes past the scheduled departure time. On the plane you will get nothing but a seat so don't expect anything else.