I did some testing back in 2013 and the 60csx held its own quite well. In these tests, the GPS was stationary with a good view of the sky and I just let it record one point per second for about a half hour, resulting in ~1800 points. The center of the bullseye is the actual location and the rings are spaced one meter apart.
With the 60csx, 94% of the points were within 10 meters and they were clustered pretty tightly (although off-center). There are "tails" at 5, 7 and 10 o'clock though where it wandered pretty far.
The Montana 600 did not show such a tight pattern of points, it was more like a shotgun blast and some points wandered very far from the target.. But if you averaged them all, you would be very close to the center of the bullseye. On the surface, it looks like the Montana is not as accurate, but if you were recording points with waypoint averaging, you might get better results than the 60csx. Still, I find this rather disappointing for such an expensive device. Statistically, 87% of the points were within 10 meters which isn't a lot different from the 60csx.
But now look at the GLO, Garmin's bluetooth GPS receiver that you use with a smartphone. It receives additional signals from the GLONASS satellites and it also records position fixes faster than Garmin's handhelds. Believe it or not, there are actually the same number of points in this image, but they are all right on top of each other. 100% of these points were within 10 meters and 75% were within 3 meters of the target. I found this very impressive for a $100 device. Garmin's new models (like the Oregon 700, Montana 700 and GPSMap 64) also have GLONASS receivers but I have not tested any. I suspect they would also beat the Montana 600.
But aside from accuracy, the 60csx really shows its age in terms of map rendering. It's really slow compared to newer devices and is not compatible with the new map formats (such as the Garmin files I posted above). Not compatible with aerial imagery such as Garmin Birdseye. The tiny screen only supports 256 colors and there's an old bug that causes lowercase text in maps to be garbled. Screen is very readable in daylight without the backlight, it's really the champ at that. But in a situation where you need the backlight, it's horrible. Very dim.
So if your main interest is accuracy in recording waypoints and tracks, I think the 60csx holds it own. But otherwise it's very out of date. I still have mine and plan to keep it though. There's just something cool about it, feels rugged and well designed. I gather they sell for ridiculous prices on eBay, so they are still popular.