They're generally fine and you'll rarely have issues.
But when they're bad... they're really bad.
In a nutshell this.
Furthermore, most individuals remember bad experiences more readily than we do the good experiences. I am reminded of my days as a sale rep for a retail chain. In any customer service environment the following holds true...
Give a customer a positive experience and you have retained one customer. Give a customer a negative experience and you've lost ten potential customers. Basically, a customer who leaves with a negative feeling is going to report that to his friends who will repeat it to their friends.
The same thing can be said of MMO random grouping. If you have a positive experience you say thank you to those random players and move on. On the other hand, when you have a negative experience chances are you leave and start chatting with your guildmates and/or friends about how and why the FP went badly.
This is the way we are wired, there is nothing you can really do about it...except take what you hear 'third hand" with a grain of salt.