I can tell you my experience in this field.
From the very opld versions of Outlook till Outlook 2003 and also as I can
undertstand in Outlook 2007 when you would wnat to use Outlook object model
in Visual basic to iterate the items
For each i in fld.Items
i.UserProperties("name_of_your_field").value=...
Next
if there is tens of items, e.g. 50, it will work fine, however you have to
figure out how to iterate the Excel rows with macro etc...
Be aware that if you have 100-1000 items then Outlook designers dis some bug
and if you watch Task manager and used memory, it will grow, grow grow and
grow and Outlook crashes out of memory
This is not a bug, this is a feature. Althouh Microsoft re-designs user
interface and cripples the interface users get used to, Microsoft vere
touched anything under cover because they would perhaps threaten customized
solutions that companies created. In 2007 they solved it such they added some
special Object - extra one - that is said not to have this "issue".
The issue is after all well documented at ms knowledgebase but for someone
logically thinking this way of scripting is dizzy and rather funny the to use
it in real world.
If you want really effective way of making import, use CDO library. It is
CDO.DLL that you have to have registered. It comes with some version of
Outlook automatically (2003) with some you have to install it (2002) and with
crippled version there is again missing (2007).
However if you use CDO - this works perfectly for 1000s of items withou
problems - yes, it is somehow difficult. If you want an example please have a
look at
http://filip.laburda.cz/zkouseni/zkouseni.html download the package
and have a look into Import.xls. The package is designed for language
vocabulary learning and flexible customized words+phrases storage, but
everything described is in Czech.
I wish you a good luck, it is really big big trouble to get used to all the
pitfalls of Outlook object model. It is also bad that Microsoft presents
Outlook just as email program because it is a great tool for small personal
data of all kinds.