Well I was watching YouTube right now, and had some programs open, the fan wasn't even on yet, so the computer was dead silent.
When you're doing basic stuff, browsing, email, music - the fan either doesn't turn on (most of the time) or spins at a rather low speed, so you can't really hear it. The only time that I really noticed the fan turn on was watching like full-screen HD YouTube video, but it wasn't going at full speed, so it was OK.
Now when you play games, the fan is definitely audible, and is pretty damn loud, too. But that doesn't bother me because the audio of the game drowns out the fan noise. Also, in neither situations does the computer get uncomfortably hot (compared to my brother's PC, which like almost burns me when my wrist is placed next to the touchpad while doing something intensive).
Overall, the fan doesn't turn on for regular use. If you do more intensive stuff, gaming and watching HD video - it will turn on, but it isn't experience-ruining (the noise). The 2011 i5 13" generally runs very nicely, I can keep it on my lap for hours comfortably.