I really like CouchDB. It's a bit of a learning curve, but views are really powerful once you understand how to use them. There is a module called cradle on github and npm that is really easy to work with. I haven't been able to test how fast it is, but it is really flexible (you can access your data in a browser too if you'd like).
The major issue here is which database design makes sense for your application. Do you have data that is largely key-value in nature? If so, use Redis. Do you have data where not all documents necessarily have the same fields? If so, use a NoSQL database like CouchDB.
The next worse thing to using a blocking database is using the wrong database for your data. CouchDB is managed by Apache, so you know that it's good quality, but it doesn't make sense to use it if your data would make more sense in an SQL table or a simple key-value store.
Think of your use case. Will you be more likely to want to do a full-text search, just get data by key, or get ranges of documents that have similar attributes?
