I have to second the previous comment. We've been running clusters (Sql Server and Oracle) for years and the only time these clusters fail over is when we manually fail them over for patches. They have been solid.

For a few of our clusters, I run Active/Active and as said previously, you have to account for the scenario where both instances are running on the same node. Essentially you want to be sure to spec the servers properly. For example, we set max server memory for each instance to about 45% of the total memory.

Side Note:

There was a post here some time ago where someone dynamically configured the memory on failover/startup so they could use up to 90% of the memory when the sql instances were running on their separate nodes.