More like a 3.5. I thought the prose was lovely, if a little too lyrical. The idea was fresh and I did respond to both Sam and Grace, who I thought were characterised well and are very believable.
Unfortuntely, I was a little thrown off by the idea of them falling in love so fast, even if they had essentially known each other for 6 years, I would've still liked more development, maybe some initial skepticism and shyness. That said, the majority of the book just floated, telling of Sam and Grace's time together. It wasn't until the last 100 pages or so, when they actually identified a problem and the plot got going that it began to be really interesting.
It ended on a happy note, although I don't know at this moment where the next two books in the series will be going. To be honest, I thought the ending was already pretty great.