Get a pda phone! One big convenience of pda phones - you wont have to maintain 2 address books! Address books of PDA phones can handle more extensive data compared to regular cellphones. Also, it is so much easier to back up your address book to a computer when you have a PDA phone. It sure beats having to re-enter all those phone numbers (and addresses, birthdays and appointments) when something goes wrong or when you decide to replace your unit.
How about photo caller ID - It is a lot easier to organize photos in a PDA allowing you to easily update your photocaller ID.
Battery life - for me its just about the same with regular cellphones. Of course there is so much more to do when you have a PDA phone (especially after the first few weeks from purchase!) such that PDA phone users tend to drain their's earlier.
Connectivity - PDA phones allow you to connect to the internet via wifi, gprs, cdma or gsm. You can check your email even if you're outside wifi coverage. You could even download email attachments.
And what about all those usefull software apps - I've got a dictionary, encyclopedia, a medical dictionary in my PDA phone. Some of my office files are stored in my PDA (word, excel, powerpoint documents) so I can easily reference them when needed. You also get more software support because there are more software developers for PDA phones.
Data entry - A lot of people complain about having to use a stylus to enter data in PDA phones. There are software available that displays a number pad on the screen (similar to a GSM phone's number pad) allowing you to enter data with your thumb only. Also, there are portable keyboards for PDAs that greatly simplify data entry.
Processor speed - PDA phones have faster processor speeds than regular phones. Your java applets and other programs will run faster on a PDA phone. Other regular maintenance tasks (updating/backup of contacts, organizing picture files/notes/tasks)will run faster in a PDA phone.
Memory capacity - PDA phones allow you to use memory cards with higher capacities (up to 5gigs). Regular phones use symbian OS that allow only up to 256mb. PDA phones also have higher RAM capacities (up to 125mb), regular phones only have around 6mb. I once had trouble transfering an MP3 file over bluetooth - my regular phone could only handle 6mb and the file I was trying to transfer was 9mb.
Multipurpose - mp3 player, digicam, diary, gaming console, and phone in one device.
PDA phones are not that much bigger in size compared to regular cellphones (check out XDA mini and XDA Atom). I cant live without a PDA nor without a cellphone. So I guess th best thing about PDA phones is that it beats having to carry around 2 devices.