I have been attending PFA/LAA rallies for almost 20 years, either by plane or car. Although I really enjoyed the earlier rallies, particularly at Cranfield because of the large amount of accommodation on site, I think the present arrangement at Sywell is almost as good as old (and still getting better), with apparently much less financial risk to the LAA.
As far as the booking slots system is concerned, I understand that this is required by Sywell but I have always found this very flexible. The Sywell website says:
"Please note: Slot times are local time. This slot system is not a strict or rigid system but is aimed at creating a safe and steady flow of traffic and providing AFIS with aircraft details thereby acting as a booking in system. Pilots are requested to try and adhere to their slots as closely as possible. If weather or unserviceability on the day affects your slot by more than 15 minutes and a new one is required, please telephone Sywell AFIS to re-arrange your slot"
I have never had a problem booking or changing a slot at LAA Rallies at Sywell and do not find the instructions too onerous.
So, if you have not been to a PFA/LAA rally in a while, give it a try. I think you will be pleasantly surprised.
PS I have never heard any plane arriving at the LAA rally without a slot time being turned away, although I have heard a few been told off for not bothering to read the AIC!