I wanted to see what my satnav would do with the Swindon Magic Roundabout, so I entered two cross-roads on either side and told it to calculate a route between the two.
Unfortunately, it treated the Magic Roundabout as a normal one and told me go clockwise 4/5 of the way around it rather than going 1/5 of the way anti-clockwise around the inner (big) roundabout. It also didn't display the mini-roundabouts on its map.
Google Maps did a bit better, though the directions were a tad confusing; the mini-roundabout it told you to leave it it simply listed as "Go through 1 roundabout".
On the other hand, it did know that the road around the roundabout was called "The Magic Roundabout" and could route you to that, or tell you that you were driving on The Magic Roundabout.