I was down at my friend's place the other day. Of the first 10 titles he got for his modded system, 4 do not work as intended, or at all.

1) PGR3 has the entire city of Tokyo missing. Loading any race there will hang the system. After a few trophies in, you simply cannot progress any further.
2) TH Project 8 gets into the main menu and character creation, but actually entering the game results in a crash.
3) PES6 simply does not initialise.
4) Full Auto also fails to initialise.

Now, some were bought in Sg Wang in late 2006. They are now impossible to change, given the VMY07 crackdown on pirated stuff in high tourist traffic areas. You're also not assured of a perfectly stamped disc, and many of these problems may come from the lack of any QC at all. I dunno about you, but many VCD/DVD movies artifact like hell towards the end of the show. Since it happens across different players, I'm thinking it's a disc manufacturing thing, or at least something to do with the disc.

Also, with pirated discs on the x360, we have found out that it doesn't take very many minor scratches on the disc surface to render it pretty much useless. Unlike the original xbox, or ps2, they'd run moderately scratched discs without a problem. Discs at my friend's place have always had a naturally high attrition rate (imagine a stack of discs in the open, with no covers...) but even the effort put into taking care of these new x360 ones can't help but result in minor scarring. That has resulted in a couple of faulty discs already.

But as always, YMMV.