What happens to the criticality factor when you consider a Falchion? Here, the base criticality goes from +3 to +8! This is a huge jump, and for a two handed weapon to boot. The problem I have with this spell (as it has been described-I never actually read the book), is that it makes it so that NO ONE would want to use anything but falchions and scimitars. Things that have extra large threat ranges gain a HUGE increase from this spell. Comparing the Axe and the Sword doesn't demonstrate this. Similarly, the Scyth goes from a criticality of +3 to a +8. So in a world where any cleric can cast a 2nd level spell and warp these ratios, EVERYONE will favor the X4 and 18-20 crit range weapons.
Also, the duration of the spell is pretty irrelivant. When you consider how cheep a ring of assassin sense is, (what like 12000 gp-or less if it has charges/day), the duration is easy to get around.
Another consideration: In order for a weapon master to gain the better crit damage, once per day he needs to devote 7 feats to his PrC and be AT LEAST 9th level. So you're gonna tell me that a 2nd level spell can do the same thing for round after round (and given a falchion as the weapon of choice, you can't tell me that a crit will only come up once per combat-especially once Keen and improved critical are factored in).
Not in my campaign, no way buddy.