I've had a BU-353-S4 for over a year now. Firmware is GSD4e_4.0.4-P1-RPATCH.07-GS008
One issue is that the initial lock can be off by a couple hundred feet. The lock is really quick (either good or bad lock). It's like the unit is too eager to lock and somehow gets a bad lock. I've had a BU-353-S3 for years now, and that unit never did this. The workaround is to unplug the unit, and plug back in and try again.
The other issue is that the bearing algorithm seems to have changed from the BU-353-S3. Appears that there is a very long filter on the bearing. Walk slowly. Position changes slowly. Bearing has no relation at all to the track until 10 seconds have gone by. Then reverse direction suddenly. Bearing SLOWLY gets around to changing 180 degrees. It's like there's a huge filter on the bearing.
Has anybody else seen these things? Would a firmware upgrade help?