We have it for one of the lines where I work. Most of the time it works fine, but it has some serious issues. At least once a week, sometimes more, it drops a call while I am in the middle of talking. You have no idea until you can't hear anything on the other end of the line. It reboots itself sometimes OR does an update - if you are on the phone at the time, it just ends your call. Sometimes I have no dail tone, at that point, I need to unplug it and plug it back in. You can not do anything else on the computer while on the phone or it will cause lapses in the conversation. (This is the one that hooks directly to the computer.) I have also been told from people calling in that their calls sometimes never go through.
My sister has the one that does not require a computer. She experiences some of the same issues. On hers, if you are doing something else that is using your internet connection (downloading or something) you just hear clicks throughout the conversation.
80% of the time it is fine, but it is not 100% reliable based on my experience.
Would I ever buy it for myself - most likely not.