Over the last few years, I have had the pleasure to implement many Real Application Clusters. I have worked on AIX, Solaris, Linux and with a colleague even Windows clusters. I have created a cluster that stretched 14km with Comic Relief for a system that had to be highly available. I have used RAC to create high volume transaction systems.
In the last couple of years, many more customers have adopted RAC and I think this is due to the Clusterware features that have come out with 10g. This ties the Database, the Database Instances and the failover and interconnect technologies together without the need for 3rd Party software, a significant improvement over the 9i implementation. Oracle Clusterware can be used for 3rd Party Applications. This link will show you how to do this if you are interested.