I am actually writing from one right now! They work just fine mine is unblocked I bought it in turkey over a year ago and haven't had any issues. Just buy a nice screen cover and protector of some sort that protects from dust because the stuff flys around and gets into everything.
The 3g network works fine and if you don't feel like dishin out the cash for that GPRS works great for basic Internet stuff.
Mine initially did not like the 3g network on Almadar it was going through the battery in 1/2 day. I manually entered the APN data in settings (almadar.net) and since then worked with out problems.
Ok, I confess I don't know anything about the iPhone yet. So I need to get one that is unlocked? Why?
Most iPhones bought from new are normally locked to one particular service provider and won't work unless they are reflashed to accept a SIM from any other SP, hence "unlocked".
There are some issues reflashing iPhones particularly with respect to the warranty. Have a look here: