Amazon Web Services (AWS) SimpleDB was a perfect fit - not only is it free for small fish like me but the new AWS Android SDK includes SimpleDB support. I'm not interested in writing a bunch of HTTP libraries - the AWS Android API does everything for me.
The big knock against NoSQL is its data consistency. You're not always guaranteed to get the data you're expecting. SimpleDB counters with a Consistent Read option which I'm employing. You give up a little speed but for a small app like mine it's a no-brainer. But there are no transactions to track so there's still some risk.
Another surprise was SimpleDB's support of SQl through SelectRequest. You can't do stuff like JOINs (these kinds of "advanced" operations are handled in application code) but it's convenient when you need to pull a targeted set of data.
Special shout out to the folk(s) who made sdbtool, a Firefox plug in that let's you interact with your SimpleDB account.
Here are a couple snippets from my class which handles SimpleDB interaction. Connecting is as easy as:
Properties properties = new Properties();
String accessKeyId = properties.getProperty("accessKey");
String secretKey = properties.getProperty("secretKey");
// some boring error checking
credentials = new BasicAWSCredentials( properties.getProperty( "accessKey" ), properties.getProperty( "secretKey" ) );
// note mDB is an AmazonSimpleDBClient
mDB = new AmazonSimpleDBClient(credentials);
Executing queries and putting results in a List is fairly easy:
SelectRequest selectRequest = new SelectRequest("select * from accounts where m_username = '" + username + "'").withConsistentRead(true);
SelectResult selectResult = mDB.select(selectRequest);
List resultList = selectResult.getItems();
I'm able to really take advantage of the API simplicity when adding stuff to the database. NoSQL architectures seem to be pretty great for these types of actions (as long as the data gets there!).
attributes = new ArrayList (1);
attributes.add(new ReplaceableAttribute().withName("m_username").withValue( username));
PutAttributesRequest request = new PutAttributesRequest("accounts", username, attributes);
You know, if data consistency is critical, you could keep querying the database until your data is positively there...