You're right, it has been somewhat disappointing, although it's also been found incredibly difficult to calibrate. For example, different morphologies lead to different relationships. There is also different scatter present in different colour bands so your choice of band will also impact your result.
It appears to function better at working out distances to clusters as you mentioned. There is also a recent paper by Source et al. (2013, ApJ, volume 94) which calibrates the TFR for the mid-infrared band. This band appears to have much less scatter and according to their research has provided slightly better results. There might still be hope yet!
Quote:
Originally Posted by madbadgalaxyman
There could still be serious errors in the distances and luminosities of our 'baseline' calibrators within our own galaxy, due to the very limited current range of the parallax method of distance estimation.
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This is very true. The Gaia mission should help improve that (
http://www.esa.int/Our_Activities/Sp.../Gaia_overview).
With the Hubble constant, there is also quite a bit of variation. I've seen recent studies showing results from 76 down to 67 km/s/Mpc (Planck 2013 results. I. Overview of products and scientific results, A&A, 2013).
All very interesting.
Cheers,
Af.