The most useful feature is that you can set up multiple tags for one campaign/event - this makes it very simple if I want different segmentations or metrics at once! Also there are many ways of setting parameters (e-commerce tracking), which gives me more flexibility when creating reports than other tools have so far offered; also tagging with cookies as well has been really handy sometimes too ;-) Some things could be improved though like making sure they're not adding double entries etc., but overall pretty good tool :).
Tracking conversions from adsense clicks through various channels suchsas facebook adverts / google search text links using utm tag builder was fairly straightforward after having used their free trial version first time around.