iTranslate requires an Internet connection, so the only advantage over Google Translate is being able to quickly access it from the menu bar. That's about the only positive thing I can say in support of iTranslate. It's cheap at $4.99, but still about $4 more than it should cost for what it does.
The biggest drawback is that you can't select from various definitions of words. Enter "washer" in English, and in French it spits back "machine à laver." Did you mean "washer" like the hardware part, and not a washing machine? Sorry, iTranslate doesn't care. (Google Translate correctly offers both "rondelle" and "machine à laver" as translations.)
Many of the multi-word definitions are similarly really poor transliterations. If you used them in any kind of communication (formal or informal), you'll at best get weird looks and at worst be completely misunderstood.