Thanks for your feedback ! After looking at COM I think I will avoid it
initially and go for an XLL.

I will also avoid VSTO as I am stuck using VS 2003 (long story) and anyways,
VSTO looks to me like a technology that is not quite ready. It is
essentially .NET "shoe-horned" into Office, which is COM based.

I will probably go over to VSTO when Office 12 arrives and it is truly a
..NET based product.