I'm new to fly fishing (my first summer fly fishing), and have never tried felt soles... but I wanted to throw my opinion out about the new Vibram soles that Simms is offering.
Last week I spent quite a bit of time fishing both the Madison River and Lamar Rivers of Yellowstone. Both rivers have lots of rock, the Soda Butte Creek also had some sand/mud in areas. The Madison also had some mossy areas that can make it "fun".
Over standard rubber soled boots, these things rock. Compared to other anglers with felt, I seemed to be right on par with them in the water, and out of the water, had a much better advantage, since the rubber sole can grip through mud a bit easier. They do tend to cake up a bit on my hikes back out (only on the outer edge of the sole though)... but it comes off easily with a couple of slaps once dry. One thing I liked is that the rubber provided solid traction as I got out of the river near a steeper part of the bank.
I do have the studs, but I have not put them on my boots yet. With the way I've felt in them so far, it doesn't feel like I need to. As I continue to try the boots on various lakes and such, I'll keep em in mind though. Admittedly, I haven't been on a river with a ton of moss covered rocks yet though.
Just I know this is a new product, so I wanted to throw out my opinion, and see what others who have tried them think also.
Cya,
Paul