I took a firm up on this, but only after I decided to accept the offer. I brought in my acceptance and also met with a partner who didn't have much time to meet with me during the callback. I don't know if I would have gone had it not been my first choice. Still, any opportunity to get a better glimpse of what the firm is 'really' like seems to be worthwhile (it's not like they're going to rescind the offer).