Yeah, it's been generally positive. I'd always used elance in the past, although that was primarily for development tasks, so moving to oDesk was a bit of a worry at first. If you do a time-based project you're at relatively low risk as you get screenshots every few minutes of what your staff are working on. (ie. If they're looking at YouTube videos you'll be able to see and refuse to pay them).
A lot of people will offer services for very little outlay so it's worth spending time investigating their feedback and interviewing candidates properly. I generally keep it quite informal and just try to work out if the person I'm talking to is able to string a sentence together and have a bit of banter. If they are I'll give them a week's trial limited to 10 hours or so and see what the results are like.
Virtual assistants are really low risk as the outlay is so low... at worst you lose $30 and at best you find someone that will work with you for the next year or two doing all the tasks that you don't have the time or inclination to do yourself.
In the past I've always done the outreach part of a link building task myself, although there are those that take the view that it's possible to outsource this effecively so I'm giving it a try at the moment. Will most likely YouMoz how I get on at some point 