i don't know much about nec's latest 8x drive, but the 4x is notorious for being picky about media.
and yes you may save that 40 mins of burning, but it's not like you're sitting there waiting for it to finish burning.
and let's say that even if you are, you aren't going to burn dvd-r discs everyday like you did with cd-r discs.
let me put this in perspective. my friend has 20,000 mp3 albums. ALBUMS. average that to 12 songs per album, and that's a total of 240,000 mp3s. now he used to burn 6 or 7 CDR discs per day. if you had to do that, you should have a fast burner. but in comes the DVD burner. now he only burns ONE dvd-r per day. so yeah i guess if you burn at 8x, you won't have to wait as long. but on the other hand, you just won't be burning that many discs period. my friend used to download more than he could put away. now he can't download fast enough. especially since most americans use slow lines (200K/sec), an 8x is not a requirement. It's nice. speed is always nice. it's just not a priority. 15mins vs. 7 mins. big deal.
we've reached the point in technology where we've got more storage capability than we have data to put it on to. for the time being at least, until high definition DVDs come out. by which time blu-ray will be available.
i wouldn't go with a brand that i didn't trust. NEC is definitely not a brand i trust to do DVD-R. maybe i'm wrong about this, but no one's gonna tell me that the 4x version was great. plextor doesn't do -R, which seems to be the standard these days anyway. if not standard, at least the most common (and cheaper) i don't know why plextor went that route. maybe because +R has more reliability. whatever.
if you want 8x, i'd wait around for the pioneer a07. even so, it's really expensive.