Secret Shopper was the first company I shopped for, back in 2006. Like ShopUntilYouDrop, I've never had a problem. I will add that in all these years, I've only had to communicate with them about a shop once, and I did find it difficult getting a response (had to email several times, and then finally call).
rainy, no, they normally don't contact shoppers after a shop is submitted. If you sign in, you can see your score and any feedback they might have left. I've never had a problem getting paid on time.