I have an 18" Starmaster with the Sky Tracker goto then track system and it is amazing! The goto is pretty accurate, or at least as accurate as the Sky Commander will allow (w/in the fov 95% of the time), but for me the real joy is the tracking. The joy of having a large dob tracking at mid to high power can not be overstated! IMHO, the Starmaster system is worth every penny of its price, which isn't cheap. The thing with this system, of course, is that Rick will not sell it for use on any scope other than Starmaster.
A friend of mine uses the Dob Driver II on his 12.5" dob, and is not overly impressed with it. He finds that it drifts quickly, must be retrained every time you move to another object, etc.
If I were looking at tracking for a big dob, I would either consider the Starmaster, or go with an equatorial platform. Three of them come to mind; the Johnsonian, which allows use at any latitude, the Tom Osypowsky platform (Equatorial Platforms, Inc.) and Affordable Equatorial Platforms, which run less than $400 for scopes up to 20". The last platform requires the buyer to finish it with either varnish, paint, or whatever else you choose.
I just placed an order for a Tom O. Compact model for my Starmaster 11" ELT, so I will warn you that the lead times are long. I will not receive my platform until around January, but Tom's platforms are arguably the best around.
What I like about the Starmaster system is that it is not latitude dependent (platforms generally work within 5 degrees of home), and I do not have to haul an extra piece of equipment around with me. What I like about the platform is that you do not lose the Dob motions (the Sky Tracker requires you to lock both axes), so it is more or less invisible.
Oh yeah, to answer Tom's question: only a dual axis platform will allow long exposure astrophotography, and the dual axis models are not cheap by any means! You can do some limited imaging with a single axis model or one of the alt/az tracking systems, but you are going to run into to problem of field rotation and/or drift.
Hope this helps,
Doug Culbertson
Midway, FL
