Yeah, we have experienced the same as you described. I have both a 5330 and a UCA client, and my UCA works great, I don't have issues. Though I am conscious to close the client and reopen it after say undocking my computer to go to a meeting, or disconnecting from the network, headset, etc..
Complaints we get is, echo, scratchy, headset getting disconnected, or all settings look good, but some reason the headset won't work. Sometimes changing USB ports will help, but not always. I have one user where the Message indicator will not go away even though she doesn't have any messages.
It just seems like we are struggling a lot with these and users are starting to get crabby..