There's a big difference between the UK Permit system, and the US Experimental system. Whilst both are sub-ICAO and national-only, how they are treated are quite different.
Under the UK system, the applicant is "invited" to discuss with CAA (or for smaller aircraft PFA or BMAA) the level of rigour which is appropriate to that aircraft - so at one end of the scale the older "type-accepted" microlights are pretty much left completely alone once approved and simply need an annual inspection and check-flight, whilst at the other end of the scale your privately owned Hawker Hunter or Gazellicopter, whilst still on a permit is subject to an approved maintenance organisation, pilot training and currency requirements, and so on. There is also a system of "MPD" or Mandatory Permit Directives allowing CAA a degree of control over anything that worries them (which usually seems to involve parts for Spitfires whenever I glance at the MPD book).
Under the US "Experimental" system, the applicant has to provide a basic summary of the size, shape and limitations to the FAA, and is then largely left to their own devices. The only real control FAA has is to ground an aircraft that severely troubles them, and there is no mandatory independent safety assessment.
You can debate endlessly which is the better system (and many do) - personally I do prefer the UK approach so long as it's administered in an enlightened and pragmatic manner. But then again, I live on a small crowded island where it's quite hard to kill yourself in a flying machine without it having some impact on somebody else - not so in the US where it's relatively easy to minimise 3rd party risk.
Probably the most contentious issue in the UK concerning permits is the lack of permission for some types - warbirds in particular - to fly night or IMC, which is prohibited. There's a sound argument that, say, a Hunter is far safer at FL150 or in an airway having punched up in IMC than mucking about low-level in marginal visibility amongst lots of light aircraft flying at half it's speed. I think we'll win that argument with CAA eventually, but it's taking a long time to even keep the blighters at the negotiating table.
G