Ok, I gotta chime in here...

So the frames that all the hardcore whitewater guys use are Down River Equipment, NRS, Recretec, River Boat Works, AAA Inflateables, etc...

These frames are the best. They are all bulletproof, and as for me it may be overkill but I am ok with that. There are quite a few people who do real whitewater and use a frame made from emt, however.

And the tubes...
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I personally am not a fan of the Chinese glued pvc Maxxon or Saturn tubes. They are pretty tough, though, and will last a while. But they do not compare to the Maravia, Sotar, Aire or Hyside tubes in durability or performance. The high end boats cost a little more, but if you do some homework you can put an affordable package together that will last forever and handle well.

I run a 15' sotar cat with a frame I loosely modeled on an NRS frame (I weld). And those NRS fittings are bombproof, btw.

Oh, and if anyone is still reading this there is a great deal of info about catarafts on mountainbuzz.com. check out the "gear talk" forum.