I've had very mixed results with 60p, and I've settled into a routine, where I use the slowest frame rate that is appropriate for the situation. I've only used 60p professionally for overcranking.
Mind you, I've been pretty happy with that.
But I've had mosquito noise macro blocking turn up in unusual situations. Large areas of a consistent mid to dark color can be a trigger for ugly moving patterns of artifacts. I think it handles a more complex scene better, which is counter-intuitive. A complex moving scene analyzed frame by frame will have some artifacts for sure, but I've found it mostly quite acceptable in motion and overcranked slo-mo.
What worries me most about the 60p artifacts is continuing through a workflow that compresses them again. Lots of what I shoot gets down-converted, and re-compressed to mpg2 again for SD DVD. This additional transcoding stress (which can be minimized, but rarely avoided completely for the end viewer), makes all noticeable compression artifacts far worse.

In the end, it's perfectly logical. You don't get something for nothing. OK, I KNOW you pay more for the 200 series, and I believe it is better overall, at the same framerates, to the 100 series. But obviously when you double the GOP, and cram twice as many (or more) frames into 19mbs you have to expect some compromises. I think it's safe to say that a 100 at 30p WILL have some picture quality advantages over a 200 at 60p. It's just number crunching logic, and 14bits only does so much.

I'll try to get some examples of my claims posted, but I'm in zombie land now. Got to get up in a little over 4 hours, and haven't been getting my beauty sleep at all this week.

Mr Clean vs. Mr Dirty
 

Obviously I'm seeing macroblocking but it's not TERRIBLE...

when shooting 30P on an HD100U I'm not having these compression issues.

is this all a 60P issue on the 200/250?

Yes. It's because of the longer GOP for 60p (12 frames as opposed to 6 for 30/24p)

I thought that the quality might be nearly equal considering that while there are twice as many frames, there is half as much movement in each frame. Of course, for high shutter movement, this all falls apart. 20mbit mpeg2 w/ gop12 is simply inadequate for 60p high shutter capture. Time to look at an XDCAM I gue$$

I'll drop back to 24p in the meantime.

Sony HDV uses a GOP of 15 for 1080i and we all know how well that holds up under motion. I'd almost wager its worse than the 60P of JVC.

I wonder what the GOP of the 1080P recording mode of XDCAM is.

how does it hold up?

If they can add 1080i to the firmware, why can't they add 35mbps or some sort of intra? I'm very frustrated that they didn't announce any product improvements at nab