I was running firmware 2.4.19 on the Netgear PTV3000 and my first connection attempt did not work any better than previous attempts. I checked the Netgear support site and noticed they had a newer firmware version (2.4.26). With little hope, I downloaded and installed the new firmware and tried connecting again.
It took a while to establish the connection, but to my complete surprise it worked! The video frame rate is quite smooth and the audio was clear. There was one audio stutter during my testing, but it was very brief.
With the Nexus 5 working, I decided to try the Nexus 7 (2013) again. It's working too! So it doesn't appear to be the Android 4.4 update that addressed the issues, but rather the firmware update to the Netgear PTV3000.
There is another Android tablet that I have been using for a couple of weeks that is running Android 4.2 and it works as well. That makes 4 separate devices, running Windows 8.1, Android 4.2, 4.3, and 4.4, that can all connect to the PTV3000 using Miracast.
I had not tested the ScreenBeam Kit in a while so I checked the support site and found new firmware for it (version 126.96.36.199 from November 4). Sure enough, it works! The Nexus 5, Nexus 7, Android 4.2 tablet, and Windows 8.1 laptop were all able to connect to the ScreenBeam just fine.
I will try to post some video in the near future showing how well the various combinations work, but it finally looks like Miracast is a usable technology. With Google locking down Chromecast, one of these adapters might actually be the best choice for wirelessly displaying your media.
ScreenBeam Kit - Firmware 188.8.131.52
Netgear PTV3000 - Firmware 2.4.26
Update: Here is some video of the various devices connecting to the Netgear PTV3000.
Update 2: I wanted to repeat the testing with the ScreenBeam, but there were scheduling issues in the room I used before (I wanted to use the same projector to ensure there weren't latency differences from another HDMI device), and then my Nexus 7 stopped responding to touch.
Subjectively, the ScreenBeam performs just as well as the PTV3000. Also, there isn't a noticeable latency difference when I used my TV at home with either device. I would definitely recommend the PTV3000 over the ScreenBeam because it performs just as well, it's smaller, and it uses a standard mini-USB connector for power. I have no problem powering it from the USB port on my television. The ScreenBeam gets warmer during operation and will be more of a hassle if you lose its power adapter.