I realize that much of this has appeared in various threads, but thought it worthwhile to highlight this issue for those who may have missed some of the prior mentions. Below I list some of my personal observations and experiences. I encourage others to contribute their experiences to this thread.
My only trusted source is WWV short wave radio. I live in the San Francisco area and only get good reception at night. Everything listed below has been compared to WWV short wave radio.
1. I've observed internet time errors at the USNO site of up to ten seconds.
2. I have one radio-synced watch, a Casio PAW-2000. It doesn't sync in my home office, but it usually syncs if I hang it in a window facing Colorado. One thing I like about this watch is that it displays the time and date when it last synced.
3. Junghans MEGA alarm clock: When I first bought it, it did not sync in my office all night despite sitting next to my short wave radio which was receiving a clear signal. The dealer told me that the Junghans received WWVB at 60 Khz. That's a long wave signal which I can't even receive on the short wave radio which I use to receive 5, 10 and 15 Mhz. When I moved the Junghans to the Colorado facing window it synced in a few minutes. Once synced, it has maintained itself nightly in my office. It has a readout of how many nights since sync.
4. Seiko Global Radio Wave Alarm clock QHR007G: Doesn't sync in my office, syncs about half the nights that I leave it in my Colorado facing window. Has an icon to indicate that it has synced, but the icon doesn't keep track of elapsed nights since sync and disappears in a day or two.
5. Sangean RCR-3 clock radio. I had high hopes for this one because Sangean makes decent short wave radios. It took five days to sync in my office. It doesn't keep a record of elapsed nights since prior sync. Every hour (day and night) it attempts to sync for 15 minutes - usually without success. I extended the FM antenna but that had no effect. It probably uses 60 Khz with a ferrite loop stick rather than the wire FM antenna.
6. My Verizon - Motorola Droid-X smartphone and my Garmin GPS III both are consistently 14 to 17 seconds off standard time. There have been some posts on WUS explaining that these devices have not kept up with accumulated leap seconds over the years.