It all depends. I find it a bit of a pain to have to register for an account on a new service, when I'm already registered as a customer of the original site.
And then how happy I am depends on how active the vendor/provider/whatever is.
Best: a feedback service with active reps who are actually paid to get on there to answer your questions and give udpates on actual progress on fixing issues.
Next best: same as above, but with no obvious commitment to fix issues (this seems most commmon).
Next best: no feedback at all
Worst: using one of these services and just ignoring it. Collecting and managing that feedback content is not the point. Acting on it is. If you're going to ask me to take the time to help you with my suggestions, you're going to not just lose me, but actually antagonize me if I don't get some lovin' in return.
So I would say that overall, it's not so much on the content management end that these services fall down as on the user management end.
But again, I've only used them as a customer. I don't know how well they help you manage this feedback from a content management perspective.
We carefully analyze all received recommendations and implement them. As for other site owners we hope that that our users are conscious people who need feedback service not for the promo but to introduce the real improvements.