Against my better judgment, I'm going to venture down the rabbit hole. I'm not a pitch man for stainless, but I am an unabashed fan. Are stainless frames a fad? Perhaps, but the fact that in seven years five different stainless tubesets have been developed and are in production by three separate tubing manufacturers, and after a somewhat tepid start in 2005-2007 with a few adventurous framebuilders experiencing great success building stainless steel frames many more have hopped on the bandwagon suggest it isn’t a fad.
Why stainless frame?
1) It's steel, and there are lots of cyclists who still prefer the resilient ride and “bend, not break” failure mode of a steel frame.
2) Reynolds 953, one of the stainless tubesets, has the highest ultimate tensile strength (1750 - 2050 MPa) and yield strength (1500 - 1900 MPa) of any steel alloy commonly used for bicycle frames. 1500 MPa yield strength is approximately twice the yield strength of cold-worked, stress-relieved 3-2.5 titanium. This high strength means that the tubes can be drawn with thinner walls than tubes with lower UTS and YS, making the frames lighter. As with non-stainless steels, flexy frames resulting from the thin walled tubing can be made stiffer by increasing the tube diameter using OS or double OS tubing. With a hardness of 44 HRc, in spite of wall thickness as thin as 0.3mm, the tubes are very impact resistant yet with an elongation of 14% has similar ductility to other steel alloys that are substantially softer. The assertion that harder means brittle isn’t true. Elongation is the property used to describe a material's range from brittle on one end of the scale (low elongation) to ductile on the other end (high elongation). For example, carbon fiber reinforced polymer has elongation of around 1% to 3% and is brittle. Plain carbon steels, chromoly steels, and the stainless steels all in the condition ready to be used in bicycle frames have elongation in the 10% to 15% range, which is pretty ductile.
For anyone who missed it, here are the properties of the five currently available stainless tubesets. It is worth noting that, in spite of the 44 HRc hardness of 953, Reynolds ovalized 953 round tubes to 70% of its original diameter without any sign of microscopic cracking as a test of ductility.
3) All of these stainless alloys with high chromium content are corrosion resistant. As Reynolds puts it with typical British understatement, "Under some conditions, e.g. sulfuric acid or prolonged submersion in sea water, 953 will show signs of pitting and corrosion. We assume these are not normal conditions for a bike frame." Even in a humid salt air environment and riding frequently in rainy weather, it's unlikely you'll ever see rust. A polished, unpainted surface has the appearance of having been chrome plated; you never have to worry about paint getting scratched and having an exposed steel surface rusting.
4) Like many non-stainless air-hardening steels, the stainless tubesets are suitable for brazed lugged, fillet brazed, and TIG welded construction.
All five of the currently available stainless tubesets can make light, stiff, corrosion resistant frames.
In summary, a stainless frame provides all the qualities of steel plus the lower maintenance and some would say attractiveness of a bare polished finish. Is it worth the premium over a high-end non-stainless painted steel frame? After seven years riding my Waterford it is to me, but to lots of folks probably not.
EDIT - Is stainless harder to work with and harder on machining tools? 953 certainly is, but a number of builders who have worked with both 953 and KVA MS2 and MS3 say the KVA tubes are much easier to work with.