We have been a fan of umbrella cover for a number of years now. The linked article by Liberty reflects a number of reasons why anyone with assets to protect needs the protection.
Over the years we have covered several $5M plus umbrella losses along with a number of smaller events. The cover is a must.
For personal use, i recommend first the Chubb excess cover with the Nationwide Private Client policy a close second. Both will allow excess Underinsured Motorist protection up to $10M.