As of Windows Phone 7.5 (Mango), it's an improved version of the IE9 rendering engine, and is generally not bad. It doesn't work with a lot of mobile sites that expect WebKit (as with Opera Mobile or Firefox Mobile), but it's not a bad browser by any stretch.
Unless the grandparent is on the GM of WP7 he should have the full IE9 Trident engine. Support isn't the greatest, as the vast majority of mobile sites seem to target Webkit, but it's far better than the release was. Tango should bring IE10
