The new 'If/Then' is sleeker, smarter and runs far more efficiently than last year's ungainlier model, the one that had its test-drive in Washington last fall. Its heart is bigger now, too, a design modification that assures a more exhilarating ride. But it's also true that the 'If/Then' that celebrated its official Broadway opening Sunday night at the Richard Rodgers Theatre had a number of flaws to address, and not all of them were eradicated in the months that this vehicle, by the Pulitzer Prize-winning 'Next to Normal' team of Tom Kitt and Brian Yorkey, was in the shop. The biggest issue was and is the show's narrative device, telling the story of a woman whose life goes in two directions, one focused on family, the other on career. It remains something of a conundrum, for the conceit comes across, even now, less as a scintillating invention than as an encumbrance...The bottom line: 'If/Then' is an enjoyable, beautifully sung, at times deeply touching experience, built on a structure that never completely works.