Nabokov is a better writer of novels and short stories than of theatrical prose, but these are really interesting reads - particularly the brief, lovely The Grand-dad (though it would read better as a short story than a theatre piece -- I can't imagine anything being gained by performance), and certain moments of The Event (the absurd private eye and the metatheatrical moments, along with the truly Nabokovian ending, are marvelous). Still, if you're not already a fan, these plays won't make a good introduction to his work.

The accompanying essays, from Nabokov's own lectures on theatre, are similarly interesting but not entirely in the way he meant. Mostly they read like a grumpy old man deploring anything newfangled and modern, but what's really of interest in them is the fact that, while one of the best things about The Event is the porous, malleable fourth wall that allllmost is broken... this goes completely against his own edict in The Tragedy of Tragedy. (Translator/son Dmitri Nabokov even notes, in the introduction to the lectures, that Nabokov might have expressed his thoughts differently later in life.)

A worthwhile read for a Nabokov fan, but if you're only going to read one (or two or ten) pieces of his writing, opt for the non-theatre pieces.