I tried getfriday.com for a week and then let them go. I used my VA primarily for research tasks.
One task I tried: find me a plain-text format wordlist of technology jargon. Specifically, I want a flat text file with each word on its own line. She billed several hours and provided me with websites I initially found on a google search.
When I started with getfriday.com, my access to their customer interface wasn't working. I tried calling their toll-free support number and received a "beep" rather than a greeting from a business. When I wrote in to cancel my account, the VA manager contacted me and I let him know about these issues. He was very attentive on the phone, but ultimately... I closed my account.
To their credit, getfriday.com let me close my account with zero hassle and they didn't charge me for the trial period.
YMMV.