SpyFu basically sold a dream that you could accurately find out what your competitors were up to as well as give you blueprint for creating a Google AdWords campaign with keyword suggestions. None of this is true. Their info on who is buying AdWords is extremely unreliable. I was actually with a client showing them my SpyFu report on their competitors. SpyFu basically said their competitors were not using AdWords. Then the client showed me the competitors' AdWords campaign popping up in their browser. This is was a very concrete example of bad data, but I've had a feeling that their data was bad before this. I would find discrepancies here and there between what SpyFu was reporting to me and what I was seeing in the real world, but it was difficult to really pin down if there was just a lag in the SpyFu data collection or something else going on. But I now feel that I have proof that their data cannot be trusted to match what is happening in the real world.
Additionally, their keyword suggestions fluctuate wildly from what Google presents as it own keyword data. I asked a SpyFu rep about the discrepancy between the Google keyword suggestions and SpyFu's suggestions and the rep admitted that Google is going to be the more accurate source.
Another feature that I found unreliable was their "backlink tool" that would show you other sites linking to the site you're investigating. How it works is, I'm researching Site A. I investigate the backlinks. SpyFu says that Site A has backlinks at Site B, C, and D. But here's the problem, go to B, C, and D and you likely will find no links to Site A. What's the point of showing backlinks that don't seem to link at all? If this happened only occasionally, that would be one thing, but it happens more often than not. Whether this is caused by out of date data or whatever, it still renders the tool useless.
As a final note, when I cancelled SpyFu, I was given a retention offer to keep the service at half price for life. Just thorwing that out there. If you, for whatever reason, want to use this junk, make sure your at least not paying full price for it.