I have been through two pairs of simms vibram soled boots is a year and a half. When the treads on the bottom get worn down the boots become downright dangerous. I wish that i had bought a pair of felts before they were discontinued just to use for guiding on local waters.
The vibram grip is okay but not nearly as good as felt on most in stream situations. Where the vibram shines for me is winter time fishing as well as hike in fishing. They have great traction on mud, ice and snow.
I think when my current pair is shot in a couple of months, i will try either the cloudviel or reddington rubber soled boot next.