Western white clematis is the wilder more energetic sibling to western clematis (the blue one). It grows bigger, faster, and has more flowers. It's definitely on the more aggressive side, and frequently sprouts from underground roots to send up multiple vines. This can be good or bad, depending on what you want. If you want to cover an ugly fence or wood pile, this is your vine. That said, it's not going to take over your yard and tends to harmonize well with other natives. 
 
The delicate white flowers are lovely, and this vine sports a lot of them. In the fall they develop into neat fluffy seed heads. Western white clematis is much more drought and sun tolerant than the blue variety, but still appreciates having its roots shaded (naturally it tends to sprout near the base of a large tree and grow its way up the branches).
 
Grow it at the base of a ponderosa, or in a thicket with chokecherry and hawthorn. Or by a fence, or over your mailbox, etc etc.