It is, even though they completely botched the physics on it, (Because they used the GT4 game engine.) it is still much more difficult than GT4.
If you want a REALLY good bike game for the PS2, then get "Suzuki TT Superbikes", but be warned, it simulates the Isle of Man TT, the hardest race in the world, 37.73 miles of streets, and with the fastest bikes you can't keep the front wheel on the ground when you're under 80mph.
(Don't get the 2nd TT Superbikes, it's rubbish.)