After a couple of weeks of beta testing, Copter-3.4.4 has been released. This is a release has several improvements/bug fixes.
a) prevent unwanted EKF core switching at startup
b) helicopter prevented from taking off in position hold mode before rotors at full speed
c) battery resistance calculations fixed
d) avoid unnecessary parameter change notifications to ground station
e) default power module definitions for Navio boards
f) on-board compass calibration capability bit added (tells ground stations that onboard calibration is supported)
Item (b) is the most serious but only affects helicopters. Helicopter users, especially those that use PosHold mode, are strongly recommended to upgrade.
The next most important item is (a) which affects all users. In short, in Copter-3.4 we have two EKFs running at the same time each one uses a different IMU (Copter-3.3 had a single EKF which used both IMUs). This bug fix reduces the possibility that we will switch to the 2nd EKF. In general, on a Pixhawk, the first EKF provides better results because it's using the Invensense IMU.