I don't have a sky caddie, but I do use intelligolf on my palm, coupled with a bluetooth GPS. I also have a pinseeker w/slope and the course I play most often has the fancy GPS in the cart that points out yardage to different hazards, as well as the pin, depending on if they have updated it accurately in the clubhouse that day.
Having used all three of those (at the same time), I find myself using the pinseeker for the vast majority of my ranging needs. I can routinely hit a limp flag from 300+ yards. I can usually hit my ball with it from 270 or so in (if I can see where it went), and it is the only thing of any use on the range. I still love intelligolf for scorekeeping, and keeping track of stats, but I seldom use the GPS feature. It does sometimes (rarely) come in handy, but being able to measure the distance to random, useful things (like the group in front of you .. though at the course with the GPS, it shows how far all the other carts are from yours, which is a really awesome feature, so long as everyone is riding) is something that GPS just can't match. Not to mention that most courses that I play don't have accurate GPS maps in either sky caddie or intelligolf. Sure I can make my own, but they still don't provide the same level of flexibility and (potentially) detail that the pinseeker does. The cart GPS is awesome for the most part, but I find myself barely ever looking at it when I've got the pinseeker out.
As far as accuracy, the GPS on the cart and my bluetooth one are usually within a yard of each other in terms of distance to landmarks and the front/back/middle of the green, once they settle down. The rangefinder is always within a yard or two of those as well. I have noticed that the pin location can be a good 5 yards different on the rangefinder than it is on the GPS, and the rangefinder is always more accurate.
Now I'm not good enough for most of this to make any difference at all to me, but until I started using a rangefinder, I only had very vague ideas of how far I hit my clubs. I'm now within a few yards on solid strikes (though I have far too many non-solid ones still).
To be honest, I think that either system is a great aid to golf, but I personally find the rangefinder more useful overall, and in the long run, probably the better value.