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From my reading, cooled EGR is necessary to run higher amounts of EGR. If you don't cool it you have HOT EGR gasses going into the engine and causing pinging/knock which will retard your timing and lower efficiency.
The limit of how much EGR can be used, and the point at which the highest efficiency is is determined a lot by the individual engine and its design.
For a throttled engine (gas), I see EGR as being a big benefit as it can reduce pumping losses and optimize ignition timing at higher loads.
For a unthrottled engine (diesel), you obviously don't have the pumping loss issue. However, you do have the NOx problem and EGR will help with that. Any additional benefit from EGR would have to do with the actual combustion process. I know Tas had posted some charts in my EGR thread that showed the effect on diesel engines and EGR did improve efficiency. However, I am not exactly sure why.
Yes, EGR is messy and will need to be cleaned out every once in a while. I'd suggest probably 30k mile intervals. A small price to pay for what could possibly be a big efficiency enhancement.
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