I did this several times over the summer. I flew SFO-LHR both times.

Trip 1 Heathrow Express to Paddington, Hammersmith and City line to Kings Cross, train to Cambridge.

Advantages:Heathrow Xpress train is really fast.
Disadvantages: Train is expensive, you have to lug luggage up and down stairs at Paddington (actually I don't recommend taking Hammersmith and city line because it is far away from the other tube lines at Paddington, it's easier to take the circle or other line). And I think that the time you save taking the Heathrow Express is eaten up by taking the tube between Paddington and Kings Cross.

Trip 2 Tube from LHR to Kings Cross, train to Cambridge.

The tube takes about an hour, but you only have to get on and off once. And because you are getting on the tube at the airport, you will probably find a seat for you and your luggage. It's also cheaper to take the tube than the Heathrow Express.

Expect to spend about 16 GPB for a one way ticket b/ween London and Cambridge.