I've heard they have a tendency to shift in the feeder hose (or wherever you put them) that in turn actually blocks air flow. I've also heard that they are a POS and that 'tornado' type air movement is actually not so good for combustion, or some such shizz.
<span style='color:green'>Nobody likes you.</span><img src='http://www.bmw-forums.com/forum/html/emoticons/ph34r.gif' border='0' alt='user posted image' />
<span style='color:green'>Are you experienced, little girl? I want to know what goes on in your little girl world.</span>
<span style='color:green'>I'm the man! I'm bad, I'm so bad I should be in detention. I'm the man!!!</span>