I agree with those who say yes if the price is right. I have a 1,1 2006 2x2.66GHz quad cores, with 20 GB RAM and do photo and video editing but purely for personal purposes not as a business.
Advantages over a mini are the ability to easily replace or upgrade the video card and RAM ( and to a limited degree the CPU) and have internal optical drives and hard drives.
I have 4 x 2TB hard drives and a SSD internally. The SSD is the boot drive and for applications, one for time machine, one for video work, one for still photography and music, the other has three partitions, one for Snow Leopard, one for Documents, the other is a clone of the SSD. All this internally. No power cables, no external cases, all SATA and when Time Machine fills I simply pull it out and insert a larger drive.
I do not care about Mountain Lion and the machine does everything I need at an acceptable speed so to me it is not obsolete. For what it is worth, the Geekbench score 64 bit is about 10200.
It is a great machine. If the price is right, get it.