Local storage of NCBI database in MongoDB
I've played with mongoDb.  It is a nearly schemaless data store - each record (analogous to a row) can have nearly any entry name set except for a unique ID that mongoDb uses.  In practice most of your records will have the same labels and names 

Leaving SQL for noSQL is turning out not to be a great win.  MongoDB will excel in retrieving records quickly and processing records by their content.  It has some constraints too compared to SQL.  It requires that the main index be stored in RAM, which might or not be an issue.   For very large datasets mongoDB can be configured to shard the data automatically - splitting it up over a cluster of machines for high throughput fast processing applications. 

If you need flexibility in your data record and you have a good machine (or a good cluster) to put it up on it could offer some advantages. We have a reasonable store of dbSNP on a recent MySQL machine and it performs okay.  Not thousands of transactions a second, but we can do reasonable throughput of record processing.