I've had the pleasure of owning all three. At the time I thought the G25 was revolutionary compared to what was available in it's price range then. The G27 was an evolutionary upgrade and I did enjoy it more than the G25 as I felt the steering was a bit smoother.
I then upgraded my G27 with a perfect pedal and "WOW" didn't cut it with the new brake. At the same time that I installed the perfect pedal, I noticed my local Fry's had the T500 so I figured I'd give it a shot.
I cannot begin to describe the difference between it and the Logitech wheels. I shaved a substantial amount of time on my laps, just due to the steering ability. While I would love to see a Perfect Pedal solution for the T500, I truly believe it is a great product.
Mind you I've gone through 4 of them as one was DOA, and the next 2 had motor/fan issues.
Overall I've enjoyed all three and can honestly say the T500 is worth the extra price for me (yes I'd like to see it perhaps $100 - $150 lower), whether it's worth the extra $ is up to you.