Splunk is really good at log parsing events over time. It is quick to drill in and analyze and it is quick to build a presentation layer and automate reporting. I love it for problem analysis and event management however it is not a capacity management tool.
It can be a cm tool but not a good tool for projections etc. There are many tools that claim to be cm tools but they are usually expensive and miss the basic day to day challenges of capacity management. Eg: excluding backups from day peaks, removing outliers, forward trending, accepting data from any source. Start by getting your key data extracted from reliable sources and other tools.
The charting and presentation layer is impressive and quick. It can probably do anything if you tweak it enough. I would call it a very handy tool but probably not the tool. It is not that cheap either. I have used it personally to analyze big data as well as creating knowledge from some ordinary logging. I then created some pretty cool dashboards but they were more operational dashboards.
I don't think we could afford it as a capacity tool but we can use the data it simplified.