This week I am taking an online ROUTE course through New Horizons and they are using the Cisco Learning Network hosted labs for the course. So far I have found it to be a very good tool. The IOS instances are very responsive and full featured. No errors about a feature not being implemented in the simulator (because it's not a simulator). The web interface to get to the labs is still a bit rough, but I figure that Cisco is probably working on that. As others have noted, the timer starts as soon as you enter the lab so reading the lab materials does count against your block of time. For the class that isn't too important as we have a printed lab materials, but if you buy your own block of time you will want to enter the lab, print the lab, and then exit while you read to save your time.
One of the biggest advantages to me so far has been that I have my own "pod" to do the class labs on. I find that I have "finger memory" so even if I see something done, I don't learn it until I've typed the commands myself a few times. Often in past Cisco courses I have had to share a pod of equipment which meant not always getting true hands on time.
All in all, this is a great step forward for Cisco. Now if they would please release IOU to their customers to use for adhoc labs and testing purposes they would really have something.