Time has been an issue for me, since I've had only renting acces to Imacons (I sure don't have $20k to buy one). So being able to scan 10 4x5" negs an hour has given me an opportunity to scan relatively large amounts of images in a comparatively good quality at a bargain price.
@ professional: If quality is the major concern (which it of course is for most LF-photographers), one could also consider moving up in format to 8x10...? Four times the area.
@ Noah: How was the learning curve when you bought the Howtek? Just curious... most people say it is immensely complicated, but I somehow suspect it not to be