Grant and Mathis play their roles seriously which feels right. The supporting cast, led by Judi Dench, provides comic relief and supports the developing relationship between Jack and Amy, although Jack's drunken friend William is a bit over the top. The romance between Jack and Amy blossoms as they parent baby Sarah together, about which they have some differing views.
It's ironic that child rearing is at the center of marriages, although most couples get married with no idea of how to raise children, or even if they will do it well together. It's also ironic that, while romantic passion produces children, the time, money and energy children require often destroys that same romantic passion. In Jack & Sarah the process is reversed - Jack and Amy start with child rearing and then fall in love, although without much heat and passion. If you are looking for a low-key, modern British romance, and you enjoyed P.S. I Love You, this film might satisfy you. On the other hand, if you want something faster-paced, with more romantic tension and more comedy, I would suggest Bridget Jones's Diary, Four Weddings and a Funeral, Love Actually, Notting Hill, or The Wedding Date.