I was working with a Client this week on SageCRM.com, and though this product looks similar to SageCRM when looking at the interface, there are several very large differences when you take a closer look.
In SageCRM you have an area for High Level User Configuration. This area is not available when you are using SageCRM.com. All of these settings have been pre-set and are not accessible because you do not have direct access to the server.
The .com package definitely has it place and is a great solution for many people that do not want the responsibility of a server. Just remember that there is a trade-off for the ease of use, and that is that you do not have the same level of direct access to the database when you are using .com.
As long as you know the differences, both products have their application.