I just put mine on yesterday, so what I have is only initial reactions. My rig is also the 305/555, which I understand responds very well to the EZ (and others).
The truck drives completely different. It revs freely now and there seems to be no noticable turbo lag - no waiting for the turbo to "spool up". Throttle response is immediate with a VERY noticable increase in power. I've only been on the Level 3 setting.
My main interest is in towing power and that will be tested, begining Tuesday. I should have good comparisons within a week.
Read the latest TDR magazine. One reason I went with the EZ is that it was the only one tested that did not cause the common rail fuel relief valve to routinely open, something it was not designed to do. I've also heard others that tow or are interested in dependable power (not to be confused with potentially equipment breaking ultimate power) have had years of sucess with the EZ.
If you want to be King of the Hill, don't mind breaking parts, looking to upgrade turbos and transmissons anyway, maybe the EZ is not for you.
Again, I'm on Level 3. If I go to 4 or 5, I'm sure I can break stuff too...

. I believe the TDR article points out that anything over a 40-60 HP gain is beyond the useful capability of the stock turbo.
It's a good article and I appreciate those that research and write articles that the membership can benefit from.