My comment is being taken out of context here.
The iPad GPS is a standard GPS device. It will work in wide open spaces or in the bush as well as any standard GPS device. If it can get satellite locks, like any other GPS setup, then it will work. If any GPS device is in a canyon deep enough to block view of the satellites it will not work regardless of what chip is in the device. This includes the SirfIII.
To be clear, the iPad does not need cell tower access in order for the GPS to work.
That said, depending on where you are the iPad may work better than a dedicated GPS receiver because it can augment its location via cell. Yes, this is a specious argument.
Will it work as well as the SirIII? Satellite acquisition times? Sensitivity? I don't know. I'd like to see benchmarks on that.
For in vehicle usage an external antenna makes sense regardless.
the gps in an apple products are not the same as a normal gps. turn the wifi and cell off on your phone for a few days and then try to launch a topomap app. you will be absolutely shocked on how long it takes to find you and the lack accuracy when/ if it does (1000'+)
if you use the ipad/iphone while camping etc in areas that you occaisionally get cell signals you should be fine. if you are in areas that you are a day+ from cell service it really does not work.
this is from a gps review site:
Like smartphones, the iPad is designed to use as little power as possible to preserve battery life. With that in mind the GPS in the iPad 3G is “A-GPS” meaning “assisted” GPS. This means that the GPS chip gets “assistance” finding your location by first approximating your location based on nearby wifi signals as well as nearby cellular towers. Since it will typically be getting this assistance the GPS chip itself is of lower power than those found in a dedicated GPS device and thus isn’t quite as good at pulling in weaker GPS signals
I’ve turned off wifi as well as turned off cellular data on my iPad 3G and it can still find and track my location… just not nearly as quickly as it can with “assistance” from wifi and cellular data. So while it does work without the data plan, it might not meet your expectations in how fast it can get a fix, how well it keeps the fix, and how accurately it tracks your location.