Migration Assistant is great because it's simple. It's also terrible because it's simple.
Which is to say, if your previous mac/s are all "gunked up" with old apps, pref files, and whatnots, you may decide to take advantage of the opportunity to set up a clean machine.
Unfortunately, Migration A doesn't provide sufficient granularity for this.
If you want to move things over "by hand" you can with a bit of googling find information on how to safely transfer things like your keychain, address book, ical, itunes, safari and other "core data".
I did this recently when upgrading to an iMac 27" and although there was a degree of time involved I like knowing I have a mac free from accreted detritus. (I kept a timemachine copy of the prev machine's HD so if I down the road realize I forgot to migrate/move over something vital I can always go back and get it.)
Migration A is definitely less hassle though.
Good luck!