Why would tension be reduced? Maybe slightly as the chainring is much smaller, so the leverage isn't as great... But a slack chain? That's why it acts as a chain guide.

 chain tension is from your derailluer, this is just rad in all ways.. except for the cost

 it doesn't matter if the sprocket is large or small. if you run the right sized chain and have your derailleur properly set-up it wouldn't matter if you ran an 11 tooth or a 300 tooth front ring.

 they market for 900-1200 a set. that was enough for me to say fuck that

 most places sell them for around $800. if you work out the cost of a crank, BB, chain guide, derailleur, bash guard, and everything that the guide replaces its not that bad of a price, its a little bit more, but still.

 have you been living under a rock?
it came out this year, a while ago. it uses crazy internal gearing, and allows you to run 2 high front gears, all on one tiny driver. using all the crazy internal gearing.
you get two fast gears, in one small thing, doubles as a bash guard and a chainguide.
and check out that ground clearence!

 Thanks! I never bothered to check into it. seems I will now!