I've been a member since July of 2000 and I've redeemed over 100,000 points over the years. I used to get Webcertificates almost exclusively but I've also redeemed for RFC gift certificates and Target giftcards. It's probably one of the easiest programs for earning points (5 pts. per email click-thru) but they are mediocre when it comes to crediting for online purchases.
Like PrincessKitty said, you need to save your confirmation emails for 30 days or more just in case they don't credit your account properly. I've had to email them numerous times to get the credits that they owe me but I've never had a problem getting my points as long as I have the proof.
I haven't done too much shopping thru them recently because I like the way CreationsRewards.net handles online purchases better...you forward your confirmation email and the points are credited to your account within 24 hours. Those points are available to use as soon as they are credited (unlike MyPoints, which makes you wait a month). CR also sends "paid" emails, has daily click-thru's and trivia and has a sign-up portal. Lastly, CR has redemptions in the form of Disney Dollars, VISA giftcards, PayPal deposit and check-by-mail. Now that Webcertificates appear to be a thing of the past, I find these rewards to appeal to me more than the ones on MyPoints.