I've got a Dirtworker and it's great.
A friend bought one of the Mobi ones from CRC:
http://www.chainreactioncycles.com/Models.aspx?ModelID=34761
and there's not much difference really. It's got wheels on it that are too small to be much good on anything other than tarmac and I think the capacity is a bit bigger. But, in use – it seems the same.
Can't comment on the Brand-X one, but I have no reservations recommending the original Dirtworker.
Seems like a lot of money when you buy it, but the first time you use it, you realise it's completely worth it.