1) Some circles are undoubtedly hoaxes;
2) Some circles were clearly constructed by humans (like bikers) and witnessed by others during construction;
3) According to anecdotal evidence, some other intricate circles appeared mysteriously out of nowhere in the middle of the night, in well-populated rural areas, and no one witnessed any lights in the fields, sometimes within hours. Keep in mind that the evidence is anecdotal. The believers call them "eye-witness reports," but the fact is that the witnesses are reporting that they did not witness anything.
4) According to some reports, some soil samples exhibit inexplicable microwave residue, and brittle stalks that would break if bent by a human hand or mechanical device are actually bent with strange 'nodules' at the bending point;
To jump from these facts to Alien intervention theories requires a leap of faith, or a preconceived idea looking for evidence to support it, not the other way around. Very similar to Von Daniken, who went looking for selective evidence to support his theories all over the world, or the Bush administration to support their fabricated 'evidence' to 'prove' WMDs in Iraq. These are simply a few features that cannot be explained by some scientists, or folk-knowledge.
Anything inexplicable in earlier times was attributed to Mithras, then Jehovah, then God, and sometimes Satan. Witches were burned at the stake for inexplicable phenomena. Now that Gods and Demons are out of fashion, we have latched on to Aliens.
Then we have the curious fact of these circles being concentrated around a few townships in England.
What is the advice that Sherlock Holmes would give? "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."
Ergo, there is a small band of humans living in England, possessing a technique or technology that is not recognizable to the few scientists studying these phenomena, who pass on these secrets from father to son in a generational fashion for reasons unknown to the rest of us.
"O' the good Lord some giftie gie us,
To see oursel'es, as others see us..."
~ Robert Burns