What are the benefits of using MySQL Cluster?
When I wanted to get information about MySQL Cluster, MySQL (before Sun bought it) would send out a brief questionnaire, which I wish I still had. One of the questions asked if your tables had integer keys only.

Although MySQL Cluster is ACID-complaint, it does not provide a suitable storage engine for data with compound keys. In some respects, you have to plan its usage like that of a MongoDB or Cassandra in terms of data layout and sharding.

You also have to consider the most annoying part of MySQL Cluster: Network Communication. NDB Cluster tends to be chatty among servers. Even Gig-E can be brought to its knees if doing range queries or full table scans. You must be disciplined enough to use primary keys and unique keys, especially if your application thrives on such keys.

Unless you adhere to these simple observations, MySQL Cluster will do way more harm than good.