I haven't had a single issue with correctness in my 9 months of development. That is, everything looks and operates exactly the same on the device as it is the simulator.

Performance is obviously the tricky part, along with seeing real hardware in action (GPS, 3G connection speed) and software features (Push Notifications, iTunes integration).

also note that you don't need to buy the developer program to develop and run programs the simulator. the developer program only is needed when you want things to go onto a device