OMG. this is like asking if a chef's knife is useful in the kitchen.
and the answer is... it depends. it depends on what you're doing.
Google Analytics (and I've used all of them: Omniture, Coremetrics, Clicktrack, etc.) is a basic must-have tool for SEO. analytics shows you who is coming to your site, how they came there, what they're looking at, and how they are converting. SEO can't be done without analytics.
analytics tools however have thousands (not kidding) of reports, most of which are useless. companies ask for these and the developers are obligated to add those reports. so, yes, these can be wildly overwhelming. i've rarely worked with a client (actually, never) who knew how to use analytics. they just poked around in it.
but it's hard to tell others how to use it. you kinda have to already know. i've been using analytics tools since 2002-3 when Clicktracks came out.
don't pay for Omniture or Coremetrics. Google Analytics produces the same results and it's free.
other useful (i.e., must-have) tools for SEO and digital marketing incl. Google Adwords, Google Webmaster Tools, and Bing Webmaster Tools. you use all of these together as a tool set.