The W20 is the next in the series after the WPi, I think, so it should be pretty similar with a few improvements. I don't know what those improvements are, but here are my thoughts on the WPi.
nice to have a camera you don't have to worry about getting wet. I did a face-first tumble into a Willowemoc tributary about a month ago when the bank gave out under my feet, and I caught my balance with my hand on the bottom of the river -- holding the camera. It came through that and many other trials just fine. It's also very nice to have something you can pull out when you catch a fish and not worry about accidentally dropping or dipping it in the drink.
It also opens up the chance to take underwater photos, which can be really fun. You can see several of those photos on this site (be sure to look at which camera was used).
By far the biggest problem with the WPi is its low-light performance. It sucks for handheld shots at some of the prettiest (and fishiest) times of day, unless you're taking pictures of the sky itself and not the foreground. It tends to take too long an exposure and cause motion blur, or else it cranks up the light sensitivity (ISO), which causes an annoying amount of noise (graininess) in the picture and a generally unpleasant look.
One partial solution to the low-light problem is to carry a small tripod. Also, for fish pictures, it has a pretty good flash. But you won't want that for stream shots. You can get a really small (about 4-6 inches) tripod that fits in the same vest pocket as the camera for less than $20, probably, and it will tremendously improve low-light shots. I only recently started carrying one but I consider it essential with this camera. It's also great for underwater photos, to help you hold the camera steady in the current.
One other problem worth noting is that the battery life kind of sucks. To be fair, I haven't used other compact digicams with large LCDs, so maybe it's par for the course. It shouldn't keep you from buying the camera, but you will want to be sure to charge the battery after pretty much every day of use.
The picture quality, in good lighting or with the tripod, is pretty good. It's at least average for this type of camera. But if I was going for quality alone, and didn't need something waterproof, I would get the Panasonic Lumix LX-1