I recently took a day to go out an help a former cob workshop participant (also named Greg) mix some cob for a project that he took on in his backyard. We wanted to mix a lot of cob in a short period of time, so we rented a mortar mixer to help.
Greg is building a playhouse and/or garden shed. The foundation is all urbanite, that he mortared together.
The quality of cob that we got from the mortar mixer was really quite good. We mixed it on the wet side, so that we could wrap it in the tarp and keep it usable for a couple weeks (and Greg could use a little bit each day, as he had time).
One advantage of the mortar mixer is that it dumps the cob right out into the wheelbarrow for you, when you tip the drum.
A disadvantage of course it that is burns fossil fuels, and is really, really loud. This was my third experience using one, and I enjoyed the opportunity. When building alone, it can become exhausting and demoralizing to have to mix cob with you feet every day (to be fair, using the mixer is also exhausting to the upper body, because you have to lift all of the buckets of materials up into the mixer drum). Sometimes the building is the really fun and inspiring part, and having to stop to mix every hour ruins the flow you might be able to achieve. So having a tarp full of weeks worth of cob from one day using a machine can be pretty neat.