There's no time to waste. Adding extra pages to passports will soon be a thing of the past!
The U.S. State Department announced mid-November that as of January 1—that's this coming year—they will discontinue the practice of supplying current passport holders with additional visa pages. December 31 will be last day to submit a request; after that, renewal will be the only option. So act fast if you want to save an extra $28.
In general, 24 additional pages cost $82 while renewing a normal 28-page passport will run $110. (It's worth remembering that 52-page passports are the same cost as a 28-page passport.)
The change is meant to “enhance the security of the passport and to abide by international passport standards,” though it may be a nuisance more than anything else. Some countries will restrict entry without a minimum number of blank visa pages or if there is less than six months left, so, uh, this isn't something that's worth procrastinating over.
Personally, if a passport is still valid for a good amount of time (a year or two at least!) then adding more pages right away is the best option. Thankfully the process isn't overly complicated, though it requires giving up passport use for some time. Normal processing takes 4-5 weeks; expediting it brings it down to 2-3 weeks door-to-door. So make sure there's some travel down-time available between now and the New Year.
If the passport doesn't have much life left, or if you just like the idea of receiving a shiny new government-issued ID, renewal is the way to go. It's a somewhat more expensive endeavor than adding pages, but it makes the most sense if the expiration date is looming.
Just keep in mind that expediting the process can cost $60 with an additional $14.95 for overnight shipping. This also somewhat defeats the point of adding those pages.