Once again I must recommand you have a look at the great dashboard templates on his blog and an excellent 6 parts tutorial on Chandoo's blog.
Robert just posted a review of Sparklines for Excel, where he describes the pros and cons of the Sparklines User Defined Functions.
Here is his conclusion :
"Sparklines for XL are an excellent and cost-free implementation of sparklines. They are easy to understand and easy to use. From my point of view, with Sparklines for XL, Fabrice and Nixnut filled a gap in Excel's chart engine. One gap – among others - Microsoft was not able or willing to fill with Excel 2007.
Nevertheless I strongly recommend to always carefully consider whether or not sparklines are the best choice for your purposes. Sparklines are a great concept, but only in a context where they are appropriate. Sometimes they are the best way to visualize your data, sometimes a simple Excel standard chart will be sufficient and sometimes sparklines are definitely the wrong choice (e.g. if you need quantitative precision). Don't use them just for the sake of having them".
I can only agree with this.
If you have already tried to use sparklines to build a report, you already know that these small chart are just like Lego bricks... to have them working properly, you first have to work on the global architecture of your dashboard and put the right bricks together.
Define the key values to be displayed and prioritize them, organize the layout of the report. If you look for graphically dense information, where comparison is key, Sparklines are a great option. One of the most usefull property of UDF in general, is the ability to copy/paste them across a spreadsheet, just like any formula... and no problem to align them on the grid.
Robert... I am working on your wishlist, and might include this in the next release.