I only did one solo trip so far.
There were times I felt lonely, but there were more times I felt fine. Waiting for a parade and fireworks is probably the hardest when being alone. I was doing video a lot so I showed up about 45 minutes early each time to get a front view and I think next time I will just watch at the last minute for those activities, even if I don't get the best view. TS meals can be hit or miss; so, I will probably have less of them on my next trip solo. I will be more careful not to choose group settings like the Biergarten or Teppan Edo for TS. If you stay at a value or moderate resort it is a little scary to be off the beaten path to your room especially at night as a soloist. I prefer inside rooms with a lobby if going solo; although, corridors are suspect, too. I will say I feel safer at Disney then I would in most other places. Long ride lines (Test Track, Toy Story Mania, etc.) are a little lonely. The best advice there is to do those rides with EMH mornings (if on site) or fastpass.
I did feel much more refreshed and relaxed from my solo trip. I was able to pay attention to me, myself and I. What I liked best is staying in the parks all day; I averaged 12 hour days. I never seem to be able to do it that way with family or friends. Would I do it again? My answer is yes. I know I will find myself in this situation again and I'd rather go than just stay home. I will get out for a vacation rather than staying home for the rest of my life. Stuff can happen whether you're alone or not. You do need to prepare yourself for some alone moments. Even bringing a book to read can help in those instances. I called home a few times, too.