Adding DNS options instead of replacing them has the advantage that you're more likely to maintain service when your primary (and possibly following) choice experiences problems, however rare a scenario the latter may be. You'll experience a small delay (which you may not even notice), depending on how long a particular name server is tried before the Network software skips to the next one on the list
*. But however temporary such DNS glitches may be, a slightly increased wait (with several DNS choices) is better than no service at all (with a single but balking DNS option).
My personal multiple-redundant DNS list is topped by my ISP's DNS offerings, followed by OpenDNS and
Google's public DNS. Like joemikeb I don't recall having had problems with this setup.
*) Experience plus trial and error may tell you how frequently various name servers have issues, allowing you to sequence your DNS selection accordingly, but the limited effect of this fine-tuning may not be worth your effort.