Why we recommend upgrading to IE8
IE8 is faster, more secure and more stable than IE6 or IE7.
Many high-profile websites are starting to drop support for IE6 to push people to upgrade to a more modern browser (IE6 is over eight years old). So use of IE6 will soon drop low enough that websites will no longer support some of the IE6 quirks/bugs.
Home machines are getting upgraded by Microsoft Update as it pushes IE8 out so users will be more comfortable using IE8 in the office as well.
We believe it is useful if all users in your office are on the same version of any software product. It simplifies user training and troubleshooting.

Potential Issues

It’s possible that the update will create a software conflict which requires work to roll-back to the setup previous to the update or fix a problem the update caused.

It’s possible that some websites will not display correctly in IE8. The IE team has done a lot of work to try to make sites display correctly and there are ways to display websites as IE7 does. It has a compatibility mode which you can read about on the Microsoft website - http://www.microsoft.com/windows/internet-explorer/features/easier.aspx.
It’s possible that users will be confused by the new interface.
Costs –
Roughly 30 minutes per-machine to download and install IE8.
Here’s some information from Microsoft on Tabbed browsing. It’s about IE7, but most of the content is relevant to IE8 for users who are moving from IE6.