I just did this for the Eowyn & The Nazgul Challenge over at theartorder.com.
These challenges are quite the experience. I ended up spending a couple of all nighters at my aunt's dining room table while I was on vacation. That wasn't the plan originally. I started on it the day the contest was announced. I just had problem after problem getting it how I wanted. One day not being able to draw, the next day re-working everything I did the previous day when I couldn't draw, then a day or two of 2nd guessing my entire idea, throw in a few days of completely debilitating self-doubt and there you are at 5am the night before the deadline at your aunt's kitchen table.
In the end it turned out ok. It's not my favorite piece of all time, but I don't hate it (That happens alot).
What I really need to do is stop obsessing over a finished piece. I could keep working on any one of my pieces for the rest of my life and never be quite satisfied.
So..on to the next piece. I have some actual paying gigs to take care of that are not exciting at all, but necessary to have income.
Maybe I'll have time to fit the CGHub/ImagineFX challenge in.
Anyway, go check out the lineup of entries to the challenge. There is alot of amazing work!