I've been buying nursing bras since I was around 10 weeks... my boobs have grown 2+ cup sizes already, and the first one I bought was because target was out of sports bras in my size (and I figured I'd be nursing anyway, so...)
The next nursing bra I bought was because I wasn't feeling adequately supported in my bras at 18-20 weeks and I was at a maternity store anyway... it was SO MUCH more supportive than sports bras (and no more uni-boob) or regular non-preggo bras... I got a second nursing bra - same style, different color when I went back to exchange a pair of pants - best decision ever, very happy to have these bras and know that I'll be using them for their intended purpose soon!
As for actually NURSING with the nursing bras, I'd say the nursing bras have at least one HUGE advantage over front-clasp... there are several adjustments on the band of a nursing bra, as opposed to a single setting with a front-clasp.
If your breasts are changing sizes as you nurse, I'd think you're more likely to fall out of a front clasp... maybe that's just me.
also, I dig the built-in pad/space for padding in the nursing bra, and the way it manages to be super-supportive despite changing breast size/firmness (from my experience thus far in pregnancy)