What OS do you currently use?
What applications do you currently rely on?
Migration Assistant probably isn't going to work for you without some glitches.
The only apps you'll be able to bring over are those that are Intel-based or "universal binary" (both Intel -and- PPC code). PPC-based apps won't run.
Depending on what apps you use, even some that were Intel-based may no longer run with Yosemite.
Your DATA should "come across" ok, assuming you have current apps that can read the data.
Important question:
Do you currently use Appleworks for anything?
If so, be aware that it won't run on modern iterations of the Mac OS, and that you'll have to convert your data to work with modern apps.
I think the best course of action will be to plan on keeping BOTH Macs set up side-by-side for a while, until you can get your data moved over, and until you can work out which apps can get moved, as well.
Then, move things over a little-at-a-time.
This gives you time to check and test things as you along.