I've used elance for 6-8 projects in the past. It helped me - I can't say I'd use it again today but, at the time (2004-2006) it was a good choice. Today I'd likely opt for local talent via referrals or craigslist. I'm not sure if it's because my budgets and needs are different today or whether it's because of my experiences on elance TBH.
Overall my experience was mixed - terrible all the way to excellent. I think there is a learning curve and I would suggest starting with as small of a project as you can first so that you can see how it works. I made many mistakes but by far my worst mistakes were made when I created a single post for multi-step tasks. Had I broken each of those into smaller components, I would've been able to control the quality (and budget) better.