I have neither used nor evaluated this too set, but the '28V' moniker stinks of specmanship to me. There may be real benefit, but I am not sure how much. I build battery packs for bicycle lighting and for small robot submarines, and have spent quite a bit of time looking over battery datasheets.
People generally associate higher voltage with more power, but as we all know, power is voltage times current. When all of your cordless tools are based upon the same 1.2V sub-C NiCd cells, the total available current is constant, so pushing to more voltage does mean more available power.
But now we are changing chemistry, which means that available current may be higher or lower, so the meaning of those voltage numbers becomes much less clear.
In general (and again, I don't know the specifics of the cells in question), Lithium Ion cells store more energy per unit weight than NiMH cells, and far more energy per unit weight than NiCd cells. They store roughly the same energy per unit _volume_ as NiMH cells, and again more than NiCd cells. But where Li-Ion cells fall down is on power delivery. Where a sub-C NiCd cell can be expected to deliver 20-30A under load (and in excess of 100A under load for certain 'racing' batteries), a standard Li-Ion cell of about twice the volume of that NiCd cell could only be expected to deliver 5A. The LiIon cell is perhaps 2x the volume of the sub-C cell, has 3x the voltage, and similar ampacity, so stores 3x the energy, while weighing less. But the NiCd sub-C cell can discharge much faster, delivering more power to the load.
Li-Ion is getting more powerful, and cheaper, and is much better for the environment...but I remain a sceptic.