One of the biggest advantages to using EC2 is its scaling capabilities. EC2 offers 10 different instance sizes from m1.small to cc.4xlarge (with 10 Gbps clustering capabilities), 4 different regions, auto-scaling, load balancing, high availability via off-instance storage, durability via copying, GigE uplinks, and much more. You can't get that level of features from in any other IaaS cloud I am aware of. Yes, you might pay more than co-locating yourself or leasing some dedicated servers... but that isn't exactly an apples to apples comparison to EC2.

Do you have a testing environment or multiple testing environments? Does it/Do they need to run all the time? If not, you save there. Do you like being able to spin up an entire duplicate of your environment to do environmental tests? You can't do that in normal server environments without ridiculous expenditure.