I can just relate my 15 month or so experience with Avast Free and AIS.
1. I am all over the Net continuously everday
2. Besides now using AIS I use WD and SAS real time and the following programs in batch mode; Malwarebytes, Trend Micro Housecall, Hitman Pro, ESET, Dr. Cureit and even Stinger
by McAfee.
3. Rarely and I mean rarely have these other programs including SAS and WD ever found any Malware, including Trojans, Rootkits etc that were missed by Avast.
That is good it enough for me
AIS does what it is supposed to without conflicts with other programs, is easy to configure and easy to use.