All in all it's different -- but better. Things about the fascinate that were 8/10 on the annoying scale are either 0/10 or 2/10 on the iPhone.
coming from a fascinate you won't be blown away with the screen cause the screen on the fascinate is probably the best part of the phone.
You'll find things you don't necessarily like about the iPhone... but having a phone that JUST WORKS and a company that will actually SUPPORT the phone is a great thing.
Also, look into AppleCare if you're a warranty kinda person... I dropped the Verizon Asurion and went with the AppleCare... so I'm kinda hit if I lose the phone or it gets stolen... but anything else will be handled by Apple and their superb customer service.