Its a misconception that they necessarily 'replace' a relational database..
I've used a lot of different technologies that would fall into the nosql bracket and each one is good for a specific task (as you can probably tell from my sig)
Here's what I think a Relational database does poorly (if you have enough data):
1) Search. (as in web scale faceted search). Products such as Apache SOLR offer an incredible search function on indexed data including stemming lexical search etc.
2) Key Value Lookup. Products like Riak and Membase offer a real benefit at scale compared to a relational database.
3) Master / Master setups. If you need to combine changes on two (or more) masters you get problems quickly in the relational world. Products such as CouchDB have fantastic support for that.
On the other hand there are products like MongoDB and other document stores which offers incredible write performance and a lovely schema-less design at the cost of atomicity.
And then graph databases which look so cool but which I've never managed to get my head completely around.
It's horses for courses and everything is dependent on the workload and how much you care about consistency.
I work for a web company doing price comparison and we have a pretty uncommon attitude to data -- we have a lot of data which cover's quote information for different providers. This data changes all of the time but we don't really care if we miss an update or not. We don't *need* complete consistency.
So for a lot of our requirements nosql solutions work exceedingly well and there are so many products to choose from that are *free*
Sql Server has a lot of scale support in enterprise edition -- federated servers etc. However, I've never worked somewhere with that just because it costs so much money.
No SQL gives you a tremendous bang for your buck but it's like Linux, you need people who know what they are doing and finding people with the skills isn't so easy at the moment.
Msg 3903.. The ROLLBACK TRANSACTION request has no corresponding BEGIN TRANSACTION.http://nosqlsolution.blogspot.co.uk/