A better question would be: When is Ultimate Magus not worth it? Answer: Only when you're intentionally super-optimizing to the level of a God-type wizard.

For the cost of a single feat and a single spellcaster level(along with some skill points which you were going to spend in those skills anyway) you gain an entire list of 4th level spells to screw around with. So basically you trade one caster level for adding the capabilities of a half-caster to your already full caster. And since Magic Wins D&D there basically is no downside. My personal favorite is Warmage 1/Wizard 4/UM X, but your mileage my vary.