I've been with AAISP since 2011, starting with a 20CN connection on BT, then a free upgrade to a 20MB Talk Talk connection. Recently I've moved to a 40MB FTTC connection on BT.
Over the years I've only had few problems with my connection but when there have been issues (DDOS amplification attack, speed dropping off) support has always been excellent. There were no issues in increasing my IPV4 address allocation so I don't have to use NAT and IPV6 'just works'. I can run my own email, web and DNS servers. On technical issues (DNS, domain registration) they are knowledgeable, polite and helpful especially at the point where my technical understanding runs out.
Prior to my recent move to FTTC they gave me some really useful advice which enabled me to get the Openreach engineer to do the installation exactly how I wanted it (moving master sockets using existing internal wiring).
As others have pointed out there is a cost for this efficiency and knowledge but this is only applicable if you are comparing it to the cost of a pure 'consumer' service. Other ISPs would be charging me business rates for the connection I have from AAISP and at appropriate charging levels. On a 'like for like' basis AAISP provide a reliable, technically 1st class, cost effective service. On top of all this you get a proper grown up unfiltered connection - it's a nanny free zone.