I agree with what others have said, and will throw a couple more assertions into the mix:
* Start by managing this yourself, or hire someone internally to do it. Agencies (and trada, unless you do cost-per-conversion) aren't necessarily incentivized to spend your money wisely.
* AdWords isn't a magic bullet, and depending on the vertical, can be absurdly competitive. Running Facebook ads, display ads for relevent publishers, and retargeting should be on your marketing radar as well.
* Don't "set and forget" your AdWords campaign. You won't get it right the first time. You'll be surprised by what text ads win and which ones lose. You'll need to "negatively target" certain words, and if you do display ads, really watch what placements work and what don't. None of this is possible without experimentation.
* Don't send all your traffic to your home page. Specific targeting for segments of your customer base will do much better than general messages that don't speak directly to anyone.
* Don't spend money on AdWords without knowing where your money is going. Set up conversion tracking.
AdGrok's beta program is free right now (in exchange for feedback on what works for you, and what doesn't), so you can't beat the price. We'd be happy to help get you started! Just go to beta.adgrok.com say you're from HN.
Can't vote this up enough. We had dramatic conversion success by creating specially targeted landing pages with a lead form embedded. Tying your keywords to specific, targeted content is essential to making the most of your campaign. The less a user has to hunt around to satisfy their search intent, the more easily you can make the most of their attention.
Just in the last month we've seen more than a handful of companies that have non-trivial spend and only send traffic to their home page.
"Best practices" just don't seem to be promulgated very well.
Looking forward to trying it out, as I've hit all the points in your comment. I'll see if this works for us.