I think we all need to step back and check how measuring solubles lead and concentrations would work in the real world. I see two possibilities:

1. Quality Control
a. Get the right dose, grind, shot temperature, pressure and volume for your espresso. By "right" I mean the one that you think tastes best.
b. Find the extraction numbers for this.
c. Make sure the shots you are making and/or selling conform.

2. Finding Alternatives
a. There is a hyperplane of constant solubles extraction and concentration in the "space" of extraction variables (dose, grind, shot time,etc). In other words, you can change dose, shot timing and volume in such a way that the extraction and concentration stay the same.
b. Now if taste is more directly correlated to these later figures, being able to tweak dose and shot times to either get to a target in solubles or while maintaining that target could be very useful.
c. The same can be done for brewed coffee. If both coarser grind at 4 minutes steep and finer grind at 3 minutes steep give you a 20% extraction, will they taste the same, or are there boundary and local effects we need to know about?
d. Many of Barry's and Nick's reservations presuppose that an exploration like this will produce surprises. But you can't do the exploration without having these shortcut instruments (trust me, I know, unless you want to compete with Jane Goodall in persistence, you do not want to bake pucks to get enough data to answer questions like these)

King Seven wrote: I might be very happy with how my espresso tastes, but closer analysis may suggest possibilities that may improve it further that I might not have been nudged and guided towards without cold, emotionless data suggesting so.

Replace 'cold, emotionless data suggesting so' with 'experimentation as suggested by a fellow geek', and this may well be the motto of on-line espresso fora world-wide. Or should be, at least.
Nice one, James!

Nick wrote:In most quantitatively-measured situations, you can have a severely unbalanced extraction and still nail the numbers.

To any well-trained barista, a severely unbalanced extraction due to channeling is obvious. The shot should get dumped immediately. In your cafe, if these kinds of shots are getting served to customers, you have more problems than a refractometer can solve.

To be honest, I simply don't care whether Extractmojo and the refractometer can accurately measure the constituents of such a shot. And I don't see why you should care either.

IOW, I concede your point: the system is NOT designed to confirm that a channeled disaster of a shot does, in fact, SUCK. OK?

barry wrote:The question that I think Nick is getting to is "how does the system hold up in the real world?" instead of how it has performed under controlled testing. If Barista Jane pulls a 25-second 30ml shot which has suffered transient channeling, how will that compare in measurement to a 25-second 30ml shot which doesn't channel

EVERY espresso shot suffers "transient channeling." Your job as cafe owner is to train Jane well enough so that the degree of channeling is acceptably low in shots that are served. If you and Jane both do your jobs and the degree of channeling is low, my experience indicates that the refractometer can do a very nice job of measuring both concentration and extraction yields. And then, as Jim explained, that's where it starts to get interesting. But if you never try the system because you're busy thinking about scenarios where it might not work, it WON'T get interesting!

It still seems weird to me: Nick and Barry are the last two cafe owners that I'd expect would let theoretical objections deter them from at least experimenting with this technology.

Also, Nick, the refractometer accurately measures how much mass has been extracted from the dry coffee, and therefore helps to determine the structural properties of the puck. As a result it is a very valuable aid during construction projects for obtaining the proper permits from the local building inspector.