I have been a member of Valued Opinions for about 6 years or more now. It was one of the first survey sites I joined and it obviously works for me since I am still an active member many years later.
I get emails notifying me when a survey becomes available and it always gives the estimated length (in time) of a survey and what the reward will be for taking part. It also tells you which category the survey is in enabling you to decide if you would like to participate or not.
The rewards are generally not credited to your account straight away and can range from 50p-£2 (on average) when your account balance reaches £10 or more you can choose to receive one of the many voucher options available. I used to always get Tesco or Boots vouchers as I knew that there would be plenty of use for them, but now, I always get an instant Amazon voucher code which is emailed straight to me within minutes and ready to use on Amazon straight away.
I find this the quickest way to get rewarded and enjoy spending my vouchers there. If you order a paper voucher it can typically take 28 days to arrive (and I'm rather impatient!)
Overall I think this is quite a fair survey site, you can see exactly how much your balance is and also the whole of your survey history and how much each was worth etc. If you enjoy surveys then I think this is a good site to join