I've heard different roofers talk about guarantees that run from 10 to 30 years. Are these guarantees meaningful?

Sort of. Long-term guarantees are provided by the manufacturer. They are in essence guaranteeing the shingle, not the installation. Shingles rarely fail unless something was done wrong in the installation or unless there is some kind of natural disaster. In either case, the manufacturer won't be responsible, and the odds of you collecting anything are close to nil. Besides, you have little chance of collecting on a guarantee unless you successfully keep track of the shingle type, manufacturer, installer and all the relevant paperwork for many years -- something few people succeed at. However, the manufacturer's guarantee is a reasonable gauge of their confidence in the shingle. So if the manufacturer has one product with a 15-year guarantee and another with 30-year, you have a pretty good picture of the relative durability of the two shingles. However, the best shingles in the world are a complete waste of money if you don't have them installed by a truly competent roofer.