I'm a huge fan of Geoff Crammond, in fact I have the original GP. And I have thousands of laps on GP3. But somehow I found GP a bit of a disappointment. Yes, superior driveability and graphics, but a very clunky interface and making setups was a lot of trouble for me. I just found F1 2002 a lot easier to make setups and just go racing.
GTR is fantastic, as is it's cousin, GT legends. Turn off the aids, get the realistic settings, and you can go very, very fast. You can also get in trouble just as quickly
I would love to see a modern F1 sim out there, because there hasn't been anything serious for the Formula One fan in almost three years.