No real downside. They make their money off referral and affiliate links, if you mistype Ebay or Amazon, they'll help spell it correctly for you, and make money off your purchase.

I've had it on my network and PDA for some time, and it really us quite handy. If you signup for an account you can also create shortcuts like "mail" that'll take you to your webmail account.

I used to use a pair of Level3 DNS addresses, but they got slower over the years as more people discovered them. So far OpenDNS has been great.