Spending most of my life hopping between Europe and the US, including lots of domestic flying in the states, I have to say that I find the standard of US ATC top notch. But I need to elaborate a little.
From the ATC SIDE of things it's fine; succinct, efficient, expedient and most of all, generally makes sense, save for a few odd profile descents every now and then. Yes, sometimes spat at you quicker than a rapper on speed, but you are always welcome to ask for a repeat.
It's the air side of things in the US where the most non standard, bog slang is to be found, as every guy out there is trying to sound cooler or more colloquial than the next. But, once more, that's the way it is, doesn't bother me, I find it a comical aside to my daily job.
Having dealt a lot with US military controllers and drivers during a previous life, I can clearly see where the need on the civilian side for colorful phrases and image were birthed and/or adopted, but the difference being that in the US military, everyone comes from within the same system.
I find UK ATC great too, orderly, clear and generally at a pace that accommodates everyone. I don't always understand why you have to inform each new freq of your clearances, but hey, that's how it is.
I picture the brave and the free trying to out slang each other in their version of the aerial rodeo, while all the queen's men like to see who can inflect the most ennui into their radio calls between sips of tea.
Once more, that's just how it goes, and I find it adds color to the otherwise dreary grind of interactions on the airwaves elsewhere in the world.
Many years of Africa and the Mid East make me most thankful for the great controlling on both sides of the pond.