According from almost everything that I read the ADATS is yes, expensive but no it's not ineffective, by the contrary - The ADATS is considered to be a very effective system against low flying aircraft, specially against helicopters. The ADATS missile is very fast with a top speed of Mach3, it's very precise, has a very powerful warhead and due to it's guiding system is immune to decoys and/or countermeasures such as Flares and has a decent max range of 10Km (for a mobile short range SAM). I would say that the ADATS is for example much more effective than the Stinger missile. But of course this can be disputed since the ADATS never entered in real combat.
The ADATS initially had some reliability problems (when initially developed in the 80's) but these reliability problems where mostly solved.

Anyway the reasons why Canada retired (NOT canceled) the ADATS resumes to the following:
- Too expensive! Canadian Armed Forces are facing some major budget cuts and one of the systems that the Canadian Forces decided to cut/retire was the ADATS, due not only to it's high costs but also due to the idea that an Air Defence system is currently unnecessary for Canada - an idea which I disagree, since Air Defence systems will always be important, since there will always happen situations where ground forces will be attacked by air forces, even when those ground forces are supposedly protected by large or superior air forces - Just look at a situation where French UN troops where attacked by SU-25s in Ivory Coast.
- An "orphan" system - Canada is the only country that uses or used the ADATS (besides Thailand which only operates 2 fixed ADATS launchers) and as such this makes the ADATS system even more expensive specially if there's future desire to upgrade it.

Despite the ADATS is no longer in service I still would like to see it modeled in SAM simulator and well, asking/dreaming doesn't cost anything, does it...