Any creature can cross it without a chance of being immobilized, as long as they don't end their turn in it. It's a 5x5 area, so you need to be able to move at least 11 squares to get across it entirely, of course, so if your speed is 3 or less then you won't be able to cross it in one turn, even running... but otherwise it's potentially very easy to avoid the immobilize effect.
Anyone have much play experience with Web and can say how effective it is, as long as the initial immobilize hit doesn't land? That is, for targets initially outside the web or for targets moving through it?
I realized that I was forgetting about the ability to take a double move, doh. If monsters didn't have the ability to do that, then web would be an amazing power than it is as it would always immobilize anything slower than speed 10. Still at least it provides an barrier to speed 5 creatures who are unwilling to run or who are starting more than 2 squares away. It would be a lot more effective if there was a way to inflict mass slow or mass daze on a group which would render them unable to to safely move through the web.
On closer review of the wizard, I'm still wondering how effective he is in his role of a controller. Our party's wizard has been nothing more than an wimpy striker with above average minion killing capacity and rarely disrupts the monsters plan of battle.