Google’s Conversion Tracking is a really great tool to track the success of your AdWords campaigns. With almost no effort (you only need to implement a java script tracking code into your website) you get the conversion rate and CPA for each campaign, ad group, ad and even every single keyword. And it’s free.
There’s no easier way to see whether a keyword leads to conversions. Using your own tracking can get quite complex when buying tens of thousands of keywords.

However there are things you need to know before trusting Google CT without double checking. Just yesterday I read an article at MindValleyLabs warning that, based on their experience, Google would track about 20-30% less sales than their more rigorous internal tracking systems. That’s about the same figure we’ve experienced at Spreadshirt when first using Google CT. I think the reason for that difference is simple though. Let’s have a look at the AdWords Help Center:

In addition, remember that Google AdWords conversion tracking only reports conversions that occur within 30 days of an ad click. If a customer converts after the 30 days have passed, we don’t report that conversion. When viewing conversions for a specified time period, note that conversions are assigned to the date on which the ad click occurs, not the date on which the conversion occurs.

I’m pretty sure this explains most of the difference between Google’s tracking and internal tracking at Spreadshirt as we do not assign orders back to the date of the click.
Therefore, when you’re looking at the stats of a period that lies a few days (better: weeks) back the difference will melt down to only a few percent.
So please don’t jump to overhasty conclusions based on yesterday’s stats. Give it a while and look back later in order to get valid results.