In my case Alt Hold worked always really nice, within 1 m in calm conditions, and still does.
But one incident scared me a bit. A few sec after switching to Alt Hold in about 5m, the heli dropped to the ground. My first thought was a glitch caused by emf from the ESC. After I calmed down, I went for two other trials and like clockwork the same glitch again. But after the dropdowns I resumed flight with Alt Hold and it worked fine as always.
After a bit of analyzing the logs I came to the conclusion the glitch is caused by a kind of ‘Alt error reset’ after the GPS status goes high.
If you are in Alt Hold without GPS log ( Alt Hold needs no GPS log ) then as soon you get GPS log, the GPS status goes high, and the Alt error drops the same amount as your altitude to hold. If you in 5m the Alt error drops to -5 if you in 2m it drops to -2.
So everything works fine as long as you have GPS log before switching to Alt Hold.
I think there needs to be looked in to this issue, to reduce the further scares :-))