However, one thing to look out for is a new feature which could potentially add hundreds of pounds to your mobile phone bill! iOS 9 has something called Wi-Fi Assist - suppose you were connected to a Wi-Fi system but the signal was weak and occasionally dropping out. In such instances, the phone will use your mobile data to supplement it. This of course eats into your data allowance; you may be lucky and have an unlimited mobile data plan but the vast majority of people do not. A typical allowance is quite small, commonly only a few hundred MBytes, and additional data can be expensive - very expensive if roaming. Watch a few minutes of video and it could easily cost you £10, £20 maybe even £100.
By default, Wi-Fi Assist is turned on. Almost certainly you will want to switch it off. To do so, go into Settings > Mobile Data. Scroll to the very bottom of the screen and move the Wi-Fi Assist switch to the off position.