It would nicely explain hierarchy problem (why Plank length for unification of 3 forces - nuclear, gravitational and e/magnetic is 1.6x10-35 m, while for electroweak unification is only 10-19 m).
So, in one of our recent work we experimentally shown that this is impossible, (virtual graviton having 2 more extra translational degrees of freedom) because that would slightly modify Newtonian inverse square law in the way that is inconsistent with our accurate measurements.
So, contribution is more negative than positive and now this "twist" of string theory is under modification. Hierarchy problem thus still remains unexplained.
Currently I work on explanation of origin of inertial mass, existence of bosons and fermions, and on origin of De Broigle waves for massive particles (electron, etc) using not "trings" but "massless waves" approach.
Also I am involved in some consulting work on multilayered x-ray nanooptics for x-ray litography.
My past work was in x-ray lasers, x-ray spectroscopy, high temperature plasma and atomic physics.
Basicly, string theories simply give more degrees of vibrational freedom to classic objects - strings and membranes - and then see what the quantisation of those degrees (allowed modes of vibration) mathematically results in - if the modes of vibration "behave" like spin, electric charge, barion charge, etc.
"Curled up" simply means that they are not translational but rotational and vibrational spatial degrees of freedom. Usually (but not always) "radius" of those dimensions is considered to be around Plank length, thus experimentally completely invisible directly.
While it is too early to claim that "next floor down" consists of strings or waves or anything else, there are definitely some oscillating entities "down there" (either "inside" of elementary particles or participating in their transformations) because many poorly explained relations in properties of matter are inherently entangled for oscillating objects.