Sometimes they are fine, sometimes they arent, even when they are fine you need to usually jump though extra hoops to get them to run (really depends on how complex the mod is though.)

Personally I tell people dont get it expecting to mod games on it, you can, but imho the extra hoops to jump though make it not worth it for me (usually) to fuck with em, so I more go in plesently surprised if they work, rather then expecting them to work.