I have used the Werner Advantage standup paddle extensively for down river trips in Native Watercraft Ultimate 14.5 Kayaks and in 16' Buffalo Canoes. This paddle works well used from kayaks upon which you can stand, and also from canoes provided you add more length to the shaft. The Ultimate 14.5 x Werner Advantage make a superb combination in many rivers calm enough for standup paddling. Werner suggests paddle lengths based on surf/touring in the ocean (add 8" or 10" respectively), but I find adding more length to be appropriate when paddling from boats rather than boards, unless your arms are very short. My arms are gibbonish in length, so at 6' tall I went with a long version (84"), and I'd prefer another 4-6" of length (I like my reach to be fully extended while standup paddling - folks who prefer to keep bend in the arm will do well to add 10-12" to their height, and those with short arms perhaps a little less). The one-piece is only available up to 85", and I don't think the extra 5" you can add with the adjustable version (up to ~90") is worth the locking studs along the shaft that may cause reduced comfort to your hand over lengthy trips.
This paddle is broad and aggressive, and will get the job done in moderate riffles on class II waters (so long as you can actually stand up!). I find that it doubles decently as a sit-down paddle during streatches when you cannot stand up or need to give your knees a break - the only drawback can be troublesome overhanging branches grabbing at the long handle while you shoot down riffles. It'll do for long streatches of sit-down paddling if the boat's right and you don't bother to bring another paddle along (though I don't recommend being without a backup paddle). In rivers with long, still pools, it will behoove you to bring your standard kayak or canoe paddle should the wind pick up.
The fiberglass blade is very resilient, mine having withstood many hours of knocking against limestone and gravel. The shaft is very strong, but light and well-balanced. The plastic handle grip seems cheap-ish on first inspection, and the seam can be cast a little off on some specimens, but it holds up very well and doesn't wear on your hand during long trips.
If I paddled ocean, marshes, mud flats or lakes, I'd probably go with a carbon paddle. The extra money is likely worth the even better balance and reduced weight, but carbon paddles shred in rocky down river use. This paddle is too expensive to be considered a "cheap" alternative to carbon standup paddles, but it owns the niche of down river standup paddle so far as I can tell. In short, I'm not a hardcore weight lifter, and I've covered hundreds of miles down river with this paddle. The only thing the Werner Advantage can't give you the edge on is a windy day, when standup paddling just doesn't cut it.