My last cellphone was the E61 and my girlfriend has the E61i. It's an awesome phone but still on the big side. Adding GPS and a smaller size (ie the E71) would make it a super device. After using the E61 I can never go back to a non QWERTY cellphone.
With regards to iSync on the Mac you'll have the ability to transfer contacts and appointments but you'll not be able to backup your sms's or install applications like you can using the Nokia software in Windows. It's limited but if your needs are basic (contacts and appointments) like mine then it should do the job fine.