Very happy with my AE300. I'm sure you read about my AE100 saga about 6 months ago so I wont got into it again. For me the big improvements were:
Fan Noise: My AE100 died of heat soak on the panels. (ie mucho doscolouration) The quieter fan on the AE300 means I can run it on high permanently which will hopefully prolong the life of this PJ! I was used to and happy with the volume of the AE100 fan on low and the AE300's fan is the same volume on high.
Smoothscreen: From my viewing position(1.8xScrW) I could see screendoor on uniform areas of colour (sky clouds etc) on my AE100. On the AE300 thats gone, just a little bit of FPN. Didn't make a huge differance to acceptable ScrW ratio IMO. ie I would happily sit about 2ft closer but no more if I had to, which in my room takes the acceptable ScrW ratio to about 1.6. When the AE300 first came out some people were saying 1.25 ScrW is fine. Not in my opinion. That said, I wasn't very happy with my AE100 at 1.8 whereas I'm perfectly happy with my AE300 at 1.8. ie to cut a long story short

big improvements in PQ at a reasonable distance from screen but don't expect to be able to put a new row of seating in front of your current seating position.
As for the AE500. Theres a thread over on Avs where a lot of guys who went from a 300 to a 500 and have had their Hi-Def connected to both have said that they are very surprised that the higher res panels of the AE500 have not made a huge differance to PQ like you would think. They've been saying that HD looked 99% as good on their AE300's. So the higher res panels are seemingly not making a huge differance on hiDef for those that have it. We don't even have it and so the only benefit to us of the panels seems to be smaller pixels but as I said the smoothscreen of the AE300 does a fairly good job on the screendoor front provided your sitting at a reasonable distance.
It has a quieter fan again than either the 100 or 300 but it does still seem to suffer from the same problems as well ie VB, FPN etc.
The major increase in contrast on the AE500 is gained through a systen called AI I think which dynamically adjusts the brightness of the bulb. Problem is that you have to have the bulb on high to use this system and thus you have to sacrefice 3000hrs of bulb life. If you leave AI off like most over at avs seem to be doing the improvement in contrast is not as dramatic. While still a bit better than the contrast of the AE300, I don't think the differance would be that noticable. As I said noticable differance if AI is switched on but do you want to sacrefice 3000 bulb hours?
Still a tough decision given that the AE500 is only 270 quid dearer. If it was another hundred or so I'd probably definately say, "save the dosh and get the AE300"......but only £270 quid.....HHMMmmmm
