I just finished upgrading an Enterprise 3.0 installation (3.0 policy server+hub and clients) to 3.5.1. There is also the policy piece, which has its own compatibility issues, so I'll discuss it too.
The 3.0 clients work great with a 3.5.1 policy server and report correctly to the 3.5.1 hub. They work well with the 3.5.1 policy, although you have to test to be sure.
3.5.1 clients work with the 3.0 server. They should work with the 3.0 hub but I have not tested it personally. The 3.0 policy may break the 3.5.1 clients because of a minor comma issue in the syntax (basically you can't say
"promiser", attribute => "value";
because that comma is invalid).
If you need to upgrade, I would suggest upgrading the hub's policy to what comes with 3.5.1 (especially the update.cf in the top-level directory) and test that with both 3.0 and 3.5.1 clients. Then upgrade the policy hub to 3.5.1. Then upgrade the clients in "waves" to make sure any issues only affect a small subset of the population.
I hope this is helpful and please let me know if any of it is unclear.