Some of the best modelling knives I have used, stay sharp for a fair amount of time even with all the scoring and edge cleaning I do with the blade (blasphemous I know!).
The set of 10 I had made 2 large armies, and 5 mini armies and still have the one last blade going!
These are a great buy and good quality to get any modelling job done.
For those who know the X-Acto knives, they'll know how good they are. These blades are certainly stronger and more durable than their normal grade counterparts. Highly recommended.