Hi UWTechie,
We are currently doing a similar exercise, evaluating certain bi solutions. (Bear in mind, we currently use both the Cognos suite as well as MS Office, and I am a definate fan of Excel functionality).However, my findings so far have been as follows:
Cognos has less limitations re size, i.e Excel can only handle +- 65 000 rows of data, and Access is limited to 1Gig tables. Cognos is capable of handling much greater volumes as well as being more stable in a DW environment.
Also, Cube building functionality is a definate bonus, but can be limited by development time when additional dimensions need to be added, i.e. Ad hoc queries are easier to perform with Access, (this could be simply because I find the VB and SQL capabilities of Access easier than that of Impromptu), but Cognos appears to process results faster.
Downside of Cognos (in my opinion): It doesn't have the presentation capabilities and functionality (i.e Pivot Tables) of Excel. Also, it is less "user-friendly" and requires more "technical" knowledge. During "development phases", the creation and RE-creation of catalogs can be an irritation, but once stabilised, it is definately the better of the two solutions.
In terms of costs, Cognos is much more expensive (but I think this is an economic factor in that Microsoft is widely used in South Africa, while Cognos is limited)
On the whole I would recommend Cognos from a DataMining / Analytical perspective and the Excel add-in allows for the presentation functionality of MSOffice. However, if your data is small enough to analyse with MSOffice, rather go that route.
My 2 cents worth, hope it helps X-)