I did two 30 min runs of my 1999 JGTC Suzuka recreation, and both were great races, albeit for different reasons.
Run 1 was on Intermediate difficulty in heavy rain - unfortunately, however, I forgot to detune the McLaren F1 GTR and so it was much quicker than everything else (790 PP instead of 660 PP). However, the game's massive rubber-banding (even though all 'boost' options were switched off) ensured that I was still able to win, not least because the lead AI (McLaren) pitted for no apparent reason at the start of the final lap. It was, however, a tight finish as the lead AI came out of the pits about 12 s behind me, and was catching me hand-over-fist. I wasn't helped by the fact that I'd changed the timing to evening, and ended up doing the last 3 laps in pitch darkness, scoring 1.5 s-worth of penalties but somehow managing to win by 0.7s.
Run 2 was on Professional difficulty in heavy rain - this time I was in the McLaren F1 GTR (first time driving it!) detuned to 660 PP in line with the rest of the 19-car grid. This time, it was just a pure driving battle against the AI. I had caught the leader - the Castrol NSX GT500 - by the start of lap 9, but only passed him for the lead in the S Curves on the final lap (lap 12). It was a clean pass and a really entertaining battle, and I held on to win by just 0.8s.
Chasing down the #36 and #37 Castrol TOM's Supras at Suzuka in a McLaren F1 GTR
As I've mentioned before, Custom Races offer so much potential but there's still some notable problems, not least that it's very difficult to engineer a race where fuel and tyres are used evenly across a grid with multiple different models of car. The rubber-banding does create fun races but at the expense of being blatantly applied at times - as was the case in Run 1, there should have been no chance of me winning, and yet thanks to rubber-banding and nonsensical pit strategies, I was able to beat a car that posted a lap time some 8 seconds quicker than my fastest...
But, the realism of these Custom Races is, for me, the absolute best experience of GT. And driving that McLaren F1 GTR is an absolute blast!!