I use a bunch of different things. You definitely (IMO) want something on the web where you can at least put and access contacts. I have Outlook on my notebook. I have a Google account and a Yahoo account. I have software specific to my phone. I have stuff in our home accounting software. All of these things can be made to talk to each other, but some of them don't like it much.

Love them or hate them, MS is the 800 pound gorilla of the group and I can get everything else to/from Outlook. I cannot directly go between some of the others, at least not in any sort of batch/bulk operations.