I'm now running 4 of these, two are originals, and two are newer after they renamed them butler (stupid name by the way, Boveda sensor was just fine IMO). Running on iOS.
I've had the best luck calibrating at the RH and temperature I plan to use, so usually a small container inside my wineador with a Boveda 65. This requires calibrating manually in the app and not using the calibration wizard. The wizard will only let you use certain RH to calibrate. When I calibrated to 75% using the included kit, they would read a few points high when I put them in with a 65% pack to check them. After calibrating properly, they all work great, agree with each other and with my other hygros.
I have had to replace the battery in one of them just the other day after just a few months. I noticed that one of the sensors had not synced with the app for several days. Replacing the battery solved the issue and got it to reconnect. I never received any low battery notification through the app like you're supposed to. Not sure if this was just a cheap/old battery, or if these thing are power hogs, time will tell.
I've had no major issues with the iOS app. There were a couple of times when the app seemed to take a long time to launch, and the recent data would not load. I had to kill the app and relaunch. This made it pull in the data. This has only happened a couple of times. Looking at the version history in the app store, it looks like this was a widespread issue that was corrected in an update 2 weeks ago.
Now if they would just release an update that let's RH and temp read in tenths of a percent/degree, these would be darn near perfect. Also worth noting, I believe you can only sync 5 sensors. If you need more than 5, you have to use a 2nd device. This seems like something that can potentially be corrected in a future app update.
Based on my experience, I recommend these over the competition. For the price, you can't beat them. If money is no issue, go with Sensorpush.