Coincidentally, just this last weekend I upgraded an FAS940 from 6.5.5 to 7.2.2, as well as two FAS3020s from 7.0.5 to 7.2.3. (There wasn't a specific reason for doing 7.2.2 on one and 7.2.3 on the other, it just worked out that way because I talked to two different Netapp support staff about the different servers.)
All the updates went completely painlessly and smoothly. I was a little concerned about the big jump from 6.5 but have not as yet discovered any problems or interruptions in service from the update.
One thing that I think helped the update go smoothly is this time I contacted Netapp support and requested Upgrade Plans for each server, which I'd never done before. (I've just historically done the upgrades myself without help.) I was especially concerned with the 3020s because there were several firmware updates to apply and I wasn't entirely sure what the correct order should be.
Netapp provided me PDFs of fairly detailed instructions for the upgrades. You can also have them provide a Downgrade Plan in the event of problems. I highly recommend taking advantage of that very helpful service, even if you're fairly familiar with he process. (It's also nice to have an official plan already drawn up to file with the Change Control teams.)