Live view just rocks with manual lenses and focusing from a tripod. As a landscape shooter I could not imagine being without it. Oh well, while I have it on my DSLR I am without it on my Linhof but then I have a 20x loupe on the ground glass.
Anyway I think live view is a great addition, all us "young" photographers that never has seen an analog camera considers live view to be a natural component of any camera. With this the Pentax 645D becomes a great choice for anyone that wants "a better DSLR", ie you can upgrade from your 135 system to this and it will have about the same feature set, you don't need to sacrifice any of the key base features everyone's expecting from a current DSLR.
On the other hand if you already own the 645D and are used to work without live view there's probably little reason to upgrade. But I do think that the features a CMOS can provide will make the 645D sellable to a wider market, and that will be good for everyone interested in this system.