Doesn't it get full with people using the PnR service ?
Regarding the P&R itself - see the below reply.
Regarding the buses themselves - the PR3 buses to the city centre regularly arrive into the city centre full and standing at peaks. Route A's a rather slow route to reach the city centre, so it primarily serves passengers to the Biomedical Campus. As a result it's typically less busy, though I'm sure Cambridge South will increase loadings.
Hasn't there been talk of an additional Park and Ride site on that side if Cambridge?
They should start work later this year on the Cambridge South West Travel Hub, which is effectively Trumpington P&R #2, off Junction 11 of the M11. Will have around 1,000 more parking spaces than Trumpington P&R, but most importantly it moves the P&R further out from the city. When it was first built, Trumpington P&R was on the outskirts - now it's surrounded by new housing. For some reason though, they don't plan on closing it after the CSWTH opens - a major mistake in my eyes, as it'd make a great green space.
Notably for Cambridge South, the council have chosen a windy route (orange) to get from the busway entrance to the new P&R, which means that it'll result in a minimum of 4-5 minutes added journey time to the existing route. So CSWTH to Cambridge South will take around 20-25 minutes from exiting car to entering station.
This is likely because the seemingly obvious route (the yellow route in the attached diagram) has had 1 singular building built in the way, presumably nobody thought of keeping passive provision.
Not sure if many people will actually choose to make a journey that goes: Home (in village) - drive to CSWTH P&R - Bus to Cam South - Train to final destination. Quite convoluted and there's zero fares integration - Whittlesford Parkway's more convenient for most trips.