That's a valid concern, but let's face it: There's an element of faith and uncertainty when you're working with any ad network.
With Tribal Fusion, for example, I don't know what my average CPM is until ads have actually run--and I don't know what ads are running until they're actually running.
With other ad networks, I may be able to pick and choose among ad campaigns before the ads run, but I won't know how many impressions will be served on my site.
In the end, the bottom line is what counts. I'm guessing that AdSense will deliver higher effective CPMs (and bigger monthly checks) than I get from other ad networks. If that proves to be untrue, I can cancel my account, pull the AdSense code off my pages, and go back to what I was doing before.