Fun and sweet! I liked the characters. This is told in third person, mostly from Bonnie's and Billy's povs. The interplay between the two is generally fun. I could definitely see why these two were drawn to each other, and though I think the love aspect came up a bit too easily, this is a lighter story with a happy ending that satisfied me.
Billy was the highlight. He had the charming personality along with the ruthless gunslinger aspect and could snap into it at the first sign of danger. He seemed intelligent and well read, young though not a kid, excited about the future, but living very much in the present and recognizing time is short. I enjoyed the historical quotes associated with him at the chapter heads, and I liked how he was portrayed in contrast to various popular media.
The author balanced Bonnie's historical knowledge with a need to refresh and review information that allowed the reader to follow along easily. There is a fish out of water aspect, but it's not too prominent. Her fix it attitude vs history impact rationale didn't always make sense to me (as Lancaster points out), and she seemed to handle some situations more easily than I would have expected or preferred.
Overall, a fun, quick read with a light romance on the run.