I dont really have any advice on if its a good deal or not. However, uninsured/underinsured motorist coverage can be VERY important if you are ever in a wreck. Its really surprising how much it comes up.
Uim/um coverage can be stacked on a policy. Up to three vehicles on one policy i think. Not sure of your carrier or the policy terms, or if all cars are included, but call your agent and get them to explain your policy and make sure you have all vehicles covered for uim/um coverage and at policy limits you want. I think you still have to opt out of this coverage in writing. I will say that I believe the most important coverage you can pay for is uim/um. It is your way to guarantee coverage for yourself for substantial medical injury or disability in a wreck. I do a good bit of guardian ad litem work where I represent minors injured in car wrecks. In almost all of the cases the um/uim coverage is paid to some amount and in most cases it is never enough to pay for catastrophic injuries. As most other folks don't carry enough insurance any more as policy prices and premiums have increased.