I don't use them (I make my own), but I've heard nothing but good about them. If a string is tied correctly, it won't creep, which from the comments I've heard, Berry Strings are tied correctly.

Ultracam is a string made with the highest percentage of vectran. Therefore, it stretches less then other materials and is less likely to move from excess heat. The downside is that it is more brittle, and is slightly heavier (for a given diameter) and therefore, a couple fps slower then an equal diameter of some other materials.

I like it and make all my strings from it, because I get greater consistancy by using the same string material with the least amount of stretch. Those who are trying to eek out every last fps, from their strings will usually choose something else.