I've been playing GT1 for about 3 hours. Still fun to play, even if it's missing about 300 cars from GT2. The AI is better than GT2, and he cars in the garage have better rendering. No, it's not the most sophisticated game out there today, it's a little rough around the edges, but still great fun if you like racing games.
Of course, I don't own GT3, so comparisons are moot. However, after playing it a few times here and there, I'd say you GT3-freaks got spoiled by the graphics and forgetful of what video games were like in the middle ages of gaming consoles. (Atari 2600, anyone?)
Any other good used games you can find for $5-$10?