Been using the XTR one for about 5 months, defiantly notice decreased chain slap on the frame, still some when going over rocks and roots. If you only ride smooth trials you can manage with out a front chain device, however any more demanding trails then a chain device is required.
I know this from experience, at Cwmcarn the other month i had to put my chain back on about 10 times which was quite irritating, i now have front chain devise and have never lost chain since.
Overall i think it is a really good idea that makes a noticeable difference. Good to see it will be appearing on SLX and XT later this year.
ps. I use 1*10 set up