Darthatron's patch for Emerald works excellently, but there's a.. small, I guess, bug with Counter and Mirror Coat in it.
Counter won't reflect physical attacks if they're using a type that was formerly special, and Mirror Coat won't reflect special attacks if they're using a type that was formerly physical.
In the same vein if a special attack with a formerly physical type hits Counter, then it'll reflect it, and same with Mirror Coat/physicals with formerly special types.
Considering that Counter/Mirror Coat don't get used that often in the course of normal gameplay, it can be somewhat hard to notice, and probably only really a huge deal if you like using those moves often yourself.
I don't know if this happened with the JP versions of DPSS patches, because I saw Counter a grand total of zero times, so..