How good was the HVSS compared to the VVSS?
From what I'm seeing, much, much better.
In shorthand, it was easier to maintain, more robust, better ride quality and had a wider track.
In long hand:
The system was more robust and easier to fix, meaning it broke less as well. It also meant it could handle more weight than VVSS, so when it was upgunned/armoured/funnied, the suspension handled it much better than if it was still using the VVSS.
The ride quality was much better. It handled rough terrain better cos the roadwheels stayed in contact with the track more, spreading surface pressure and keeping the track taut so it didn't come off the idler (throwing a track). Cos it was more comfortable and robust, the M4 could be driven faster off road without shaking the crew (and the suspension) to bits.
And although it wasn't the best suspension in WWII (the torsion system on Panther and Tiger was awesome), it also meant that the tank was more stable when moving than VVSS and settled quicker after a hard stop, which in conjunction to the single axis gun stabilisation, allowed the gun to be fired quicker after stopping and also on the move.
Something that few other tanks were capable of doing.