I do a combination of solo and hiking with another person (typically one). It's more solo now, since my main hiking partner moved back to Australia! If I can find someone who is as great of a hiking partner as him, I'd be happy.
I did a great deal of Northern California, Oregon, and southern Washington on the 2004 PCT by myself, and it was pretty enjoyable. Of course, there are people around, but I definitely had days when I saw no other people, or one elk hunter.
I like hiking by myself, as it allows me to set my own pace, nap on rocks when I feel like it, and not feel like I am trying to catch up to someone faster than me. As long as I have a little scotch for the evening and an Ipod, I'm all set. I never camp at night with other folks who are in the same area.
I'm planning several solo trips for later in the summer - 5 days around the Tahoe Rim Trail, 4-5 days on the Trinity Alps "High Route", and the like. Looking forward to it!