It depends on the site. Several are very reliable, meaning they've been around a long time and they pay consistently at the rate they promise. Others have less positive history. There are few things you can do to investigate these sites:
1) Start reading the threads in the Micro Earnings (
https://bitcointalk.org/index.php?board=212.0) and Services (
https://bitcointalk.org/index.php?board=84.0) boards of this forum, which will help you see what deals are being offered and what trouble or positive/negative feedback people have for some of these sites.
2) Search specific site names in the forum search tool to see what mentions or threads have been posted previously.
3) Share your experience and feedback so others can learn from what you're seeing.
Some of these sites will pay you for each activity you complete others will require a balance to grow on their site and then you can have that balance transferred (either automatically or manually when you decide) to your bitcoin wallet after a predefined minimum amount of bitcoin has been earned. Several also offer bonuses for referral activity.
