Never paid attention to this product before. From the web site, they suggest people may use it as a quick (and more expensive) alternative to the usual watercolor tubes, possibly also for airbrushing.
Personally, out of a 3$ tube of watercolor, I can make a **lot** of watercolor ink. Some comments on the web site seem to suggest the brillance of the resulting color, so maybe if you're picky about brilliant colors it may be worth it. For me, mixing the right hue myself is critical so I have to work with tubes.
Also I find getting a high density pigment watercolor ink mix (approching the opaqueness of goauche, say) requires a bit of work and tweaking. Assuming the Doc Martin ink is thin, then maybe all you're paying for is "radiance" (and I'd like to know what they mean by that).
Did you use it yet as calligraphy ink? I'd be curious to hear how that works out.