Situations where you're not a long way from a Google datacenter and where you're not heavily dependent upon Akamaized traffic.
Various big providers try to direct you to servers "near" you on the network, by looking at where the DNS query came from and doing some rough approximation from that. This kinda-sometimes-mostly works, as long as the DNS cache is sufficiently "near" you on the net. This is part of how Akamai works.
None of the open recursors currently provide a way to pass on location information for the querier to the authoritative servers, so using anything like OpenDNS or GoogleDNS will hurt the performance that you experience of services like Akamai. How much? That depends on your local network, how close you are to Google's caches, etc etc.
OTOH, if you're a small operation and have a local web cache (squid?) to lower the utilization of your uplinks, then the traffic which Akamai serves is more likely to hit the caches anyway. Whether or not that balances out, only you can determine based on trial and error and user reports.
As long as you have low ping times to the GoogleDNS servers, it's really a case of "suck it and see" and figure out if it works for you. If it does, great, you can use a free service to your advantage. If not, then you switch back and you're not out anything. It's not like this is a hard config change to revert.
[disclosure: my employer has a stance in this, I'm not an independent voice]