I have a harness, it definitely changes the dynamic of the front squat into more of a hybrid between a zercher squat and a front squat, but for people with mobility issues or just another exercise altogether, its a great tool.
I think what steven and tychver are trying to say is that the reason the shoulders hurt most of the time, is not the shoulders themselves, but a mobility issue or weakness in the front squatting movement pattern and I agree, once you are tracking a more vertical bar path, most of the pain will usually go away. The issues are very often not in the shoulders, but in the movement pattern that is much different than a back squat. Once there is any forward lean at all, there will be tremendous pressure on the shoulders and wrists regardless of how strong or mobile the shoulders and upper back are.
I like the harness for variety, but if someone just wants to front squat without the wrist/shoulder mobility issues they may run into, its very easy to just use straps ala Poloquin
http://www.youtube.com/watch?v=ipCmj8t5aXg