I've been testing Skype for about a month now and I have been impressed. I've found myself using it more and more over time, and for me that's usually the best sign that I'm really like something and finding it useful.
As I mentioned in a previous post, the Skype 2.0 video call feature is something that I've been using in my day-to-day work to communicate with other members of my team. I continue to be pleased with how well it is working. The audio and video are synced up well and the sound and picture quality has been solid. In fact, as far as desktop video conferencing goes, it is far-and-away the best "free" solution and it is competitive with the best paid solution (Polycom PVX).
I've convinced several other people to give Skype a try and have used it for VoIP calls in many cases. The quality has been very good most of the time. In fact, the audio has had a richness that surpasses phone calls in some cases. I've had two instances where the quality wasn't very good. Both times the people on the other end were running laptops (where a built-in mic and a built-in speaker result in a feedback loop that causes problems) and didn't have headsets. Also, one of those two was on dial-up.
I've tried out SkypeIn as well in order to create an external phone number for people on landlines to call me on Skype. That has worked as advertised, but I do wish that it included some CallerID functionality.
I've also used SkypeOut for calling people on landlines. The call quality has been very good. I have asked people if they have had any trouble hearing me and none of them have complained. Several of them have said something like "No, you sound very clear." I've tried this with both a Plantronics USB headset and the standard mic and speakers and haven't gotten complaints either way (although the headset apparently has better sound). The only problem I've run into with SkypeOut is when I trial to dial extension numbers, such as dialing into a conference bridge. There is a known problem (see also this link) with the way Skype reproduces the tones that are normally created by a phone keypad. With Skype, sometimes this works and sometimes it doesn't. That is the most frustrating thing I've run into with my Skype experience. If they got that fixed, I'd seriously consider replacing Vonage with Skype in my home office.
There are some features that need to be fixed or added, but all-in-all Skype is starting to win me over.