At a price of over $300, are they worth it?  Is there a distinct improvement in accuracy over regular handguards/other FF tubes for ranges out to 350 yards?

Accuracy? Well, depends on what you mean. It will cool better than many others on the market so it will keep your barrel cooler longer which will retain accuracy longer. Its also probably several times more robust than most other standard tubes so it will take more of what you may throw its way. It is also probably lighter than many so you can add other heavy items like scopes and bi-pods etc without having to have a gun that weighs a ton. So it has its advantages... If it is worth the price you need to pay should be dictated by if you actually want or need those advantages, or even feel they are advantages at all considering your shooting needs/wants.

If you decide to get one, make sure the carbon fiber tube is not offset from the center of the bore (a lemon tube!).

Detroitchris,

Please don't take offense to these questions.  I'm trying to understand the nature of your comment.

Other than the obvious defect in a FF tube that is off center, what are you inferring?  Is it  that this will affect accuracy?  Because I can't see how it would.

Or is it that PRI has made defective tubes in the past?  Or that other manufacturers have made defective FF tubes?

PRI's tubes have had the occasional offset from the barrel centerline but PRI will fix the problem if returned to them.  An offset will affect putting a SPR rail on top and even if not you'd have to make some major adjustments to any optics relying on the forend's top rail.