They don't seem to work for me.
Marque

They work fine on the DAZ basic clothes for me, but not on anything else. Is this a DAZ Studio only thing?

Wish I could help. I suggest you look for some tutorials.
Magbets are still a mystery to me and my few attemps have been very frustrating.

Well before with other products you could just click on the mag after you put the clothing on and it would adjust. Will just have to wait a bit I guess.
Marque

Here's a little tutorial that made me realise what use those "magnetise to clothing" was:

V4 Take a look under the hood and see how she ticks.

I'm using Poser rather than DS though, but it explained it to me.

I used the V4 mag poses in Poser6 and it adjusted the items.  The V4 readme file has a quick explanation.  If you have one V4 in your scene, use the V4 pose.  If you have more than one then you use the pose name that matches the figure name (i..e., V4_1 you use the 4_1 pose).  The magnet poses actually use cross-talk so you have to select the appropriate V4 figure you plan to wear the clothing before you load that clothing item.

Duh! I didn't think to look at the Read Me!
I guess I was too excited :)

Here is my first and only V4 render so far with Domino and Stylish Shoes adjusted using the magnet poses.

Attached Link: http://www.daz3d.com/support/tutorial/tutorial.php?id=1942

There is some misunderstanding about what the magnetize pose does. this has nothing to do with the standard morphs, the magnets in V4 are primarily a replacement for Joint Controlled morphs, the magnetize pose only allows the clothing to be effected by those magnets. if you read my tutorial you might get a better understanding. here is a direct link to the tutorial.