Valentine is the name of one of the central characters (played by Irene Jacob) in the film "Rouge" (1994) from Kieslowski's film cycle "Trois Couleurs." This may explain its sudden rise in popularity in France and Belgium in the last several years.
I fell in love with the name Valentine when I read 'Ender's Game'. To me it just screams cute, and I would definitely use it on my daughter, but I'm afraid it would cause some teasing. The only good nickname I can think of is 'Allie' maybe, but that's far too overused.
In the detective novels by Margery Allingham, her detective Mr Albert Campion has a sister called Valentine; however, since his real name is Rudolph, it's impossible to be sure that it's her actual name.