I've tried alternate DNS services in the past but I've never had much luck with them. They never had anything substantial to offer other than being "better" than your ISP which was a marginal improvement at best. Recently I had an issue with www.co-optimus.com not resolving properly and RoadRunner kept sending me to their search page. I've had it happen on other sites too but it was never a major issue. Finally, I asked if anyone else was having that problem and nobody did, but Nick (bapenguin) suggested I should try Open DNS. Reluctantly, I did. And I was pleasantly surprised. Not only did it fix my DNS problems, it also introduced me to several DNS features I've never seen offered before.
DNS Resolution: As the primary service Open DNS offers, they do a fantastic job. I've yet to run into the issues I had with RoadRunner's DNS servers. The Open DNS servers are quick (<15ms), and have yet to fail on me.
Dynamic IP service: I've been using DynDns.org for longer than I can remember so it took a leap of faith for me to switch my router from using DynDns.org to Open DNS's DNS-O-Matic. To my surprise (again) DNS-O-Matic is capable of updating all kinds of third party services with your IP. Since my router is always on, I have yet to change IPs since we moved so I can't really vouch for its reliability, but it always responds right away when my router reboots and says no update needed. The fact that it updates so many other services is a huge bonus and I plan on taking a good look at the list to see what else I've been missing.
Domain and Category Content Filtering: With Open DNS, you can set up content filtering in three ways. The first is a level system where "none" disables filtering, "minimal" prevents you from visiting known phishing sites, "low" and "moderate" block adult/porn and illegal activity sites, and "high" blocks time wasting sites (social networking, music, videos, etc). You can also use the "custom" level to pick and choose from 52 categories. If there are specific domains you want to blacklist or whitelist, you can do that too. Using this method, I've blocked a few ad networks that I don't care for. The only problem I've had with this is I can't get Last.fm to work while blocking doubleclick.net. I'm sure that would be an issue with any ad blocking method so I can't blame Open DNS for that. However, it would be nice to set exclusions so doubleclick.net would not be blocked while visiting Last.fm. From a DNS server standpoint, that would be difficult.
Shortcuts: I personally don't see much of a point in using shortcuts but I could see how they might be handy. Basically, I can set up a shortcut to go to my blog by typing "blog" in the address bar and hitting enter. You can also set up advanced shortcuts where you could type "wikipedia internet" and it would search wikipedia for "internet." That, I could see being more useful.
Stats: Open DNS keeps track of all your requests, if you desire, so you can see graphs of requests on your network. You can see a list of all the domains you've requested and how many times you've requested them. This could be a very handy feature for some people but personally, its not too exciting.
Ease of Use: The initial setup can be daunting if you're not a network guru. This is not something a novice should set up on their own because it could really mess up your network. However, they do have pretty good guides for people who take the time to understand what they're doing. Once everything is set up, nobody needs to know its even there. It works just like before, just safer, and more reliable. If you need a hand, feel free to ask me!
There are a few other features to play with but these are the features that stand out and can be pretty useful. Overall, Open DNS provides safe, secure, and dependable browsing. I hope they stick around because I really don't want to switch back to RoadRunner's crappy DNS servers.