My ISP's DNS server (in the UK) quite frequently goes down or slow - it's useful to have an alternative to switch to. Normally, I change to OpenDNS then change

My ISP's DNS server (in the UK) quite frequently goes down or slow - it's useful to have an alternative to switch to. Normally, I change to OpenDNS then change back when the ISP one is back up to avoid the ads. However, since changing to Google DNS when my ISP DNS was down a couple of weeks ago, I've not actually bothered to change back. (edit: having seen davidu's comment on this page, I may try OpenDNS again, sans ads.)