Many folks asked me about Sandboxed solutions and whether they are good or bad. IMHO, they are neither good nor bad, but at least they are the only option for hosting scenarios . Besides, they make more sense for me than creating custom CAS policies.
In any case, to fully understand SharePoint Sandboxed solution, check the following resources (It will be more beneficial for you if you read them in the order specified):
1. SharePoint 2010 Sandbox solutions, the definitive guide by Sahil Malik
2. SharePoint 2010 Sandbox solutions are bad by Doug Ware
3. Sandbox solutions are pretty damn good by Sahil Malik (Make sure to read the comments)
4. A Reply to Sahil Malik’s Defense of the Sandbox Architecture by Doug Ware
5. You can deploy WebParts as Sandboxed solutions but … by Sahil Malik