Microsoft's vision with PerformancePoint is that we will use it as an end to end solution with the AS cube being built by the Business Modeler and all analysis being done with the Monitoring and Analytics components (which are improving greatly with each CTP release - CTP3 looks like having some big chunks of Proclarity Dashboard functionality which is great news).
However, when I talk to people about PPS the impression I get is that they are looking at it in terms of 'I'll buy the planning bit but will use my own BI tools' or vice versa.
I wonder if some of this stems from a too rigid appreciation of the technology available here at, lets face it, a pretty reasonable price.
For example, say you go for PerformancePoint for the Monitoring and Analytics components but have no desire to use the 'planning' functionality, just have a quick think about what you might be ignoring.
At a base level, PerformancePoint Planning gives you a security controlled, workflow based (if you want to use it) Excel front ended writeback system. So do you need to use this just for planning?
Just think of all the other applications you could build that would benefit from the ability to allow users to interact with data in a controlled fashion using their favourite Excel front end and then for those changes to be easily written back to your fact tables and then into your cube.
Have a think about it.