Assuming you have assets (significant equity in your home, for instance), it's totally worth it for the peace of mind.
You're not required to use the same company as your homeowner's, but it's probably where you're going to get the best deal, and it's easy -- why wouldn't you?
Ours is with USAA. In our litigious society, I wouldn't be without it.
The umbrella is about $100, but you have to increase your auto coverage and decrease your deductibles, so overall you will probably pay significantly more to add umbrella.
But as PPs note if you have assets in excess of $500k you really should have umbrella coverage.
We used to have everything bundled with Geico and just switched to Amica---now have everything there. Umbrella cost went up slightly ($20 a year) but homeowners and auto was enough of a discount that we're still saving money.