Depending on what you're writing, the Oxford English Dictionary is either very helpful or, um, not at all. The subscription is just under a whopping $300/year, but if you need to know when and where a word was first used, and such a need occurs frequently, the OED is for you. Especially because you can write it off on your taxes (if you're a writer. Check with your accountant.).
The most cost effective way to use the OED isn't always possible: subscribe for a month (for $29.95), and in that month look up all of the words you might need. Not always practical, but if you can make it work it certainly will save you some cash.
If you haven't read THE PROFESSOR AND THE MADMAN, which is Simon Winchester's book about the decades-long compilation of the first OED, do so as soon as you can. Beg, borrow or steal it. I found it spellbinding.
The OED isn't that useful when you need to know what a word means (i.e., as a dictionary). Go to Dictionary.com for that, which fortunately has the bonus feature of a thesaurus button (although its thesaurus is nowhere near as helpful, in my opinion, as the Visual Thesaurus).
What the OED is useful/invaluable for is: etymology (although that can be found in other places); quotations (good luck finding the quotations-throughout-history elsewhere); and a date chart of when the word has been used. If those are things you need, subscribe to the OED. But, like the Visual Thesaurus, you can lose a lot of time wandering around the site, so give yourself a time limit and stick to it if you're the type who will suddenly realize they've been looking up random words for three hours.
Finally, remember that the OED is always a work-in-progress and doesn't contain every word currently used. But it certainly contains enough to keep you busy and educated about the history of words.
