As long as you're experienced, physically fit and know what you're doing hiking solo shouldn't be too much of a risk. However, you need to let somebody know exactly where you're going and you should carry some reliable means of communication if at all possible.

That said, traveling in a group makes life a LOT easier on the trail. First, you can distribute heavier gear among several people. Second, it's easier to set up a proper camp: you can prepare shelter while your buddy gathers firewood. Your companions watch your back, you have someone to talk to and different people bring different gear and skills, all of which can be put to good use.

Going solo, there are many things you can't do and many risks you can't afford to take. Hiking with somebody you trust you can negotiate difficult terrain with more confidence and go to places that might be off-bounds otherwise. I know I would never try to do any serious climbing or winter trekking in deep snow alone. Just too dangerous.

So all in all, while I enjoy a short trek solo every now and then I prefer hiking in a small group (2-3 people). YMMV,