I used to run 2 ac 110's on my 75 gallon. They were great. Lots of flow for a 75G. Kept the water very clear with BIG fish in that tank.
What I found:
1) It's awesome how you can set up the media about however you want. My favorite was filter floss on the bottom and a bag of matrix for bio on top of the filter floss. Very easy to service, and did an awesome mechanical/biofiltration job.
2) They're tanks. Mine wouldn't die, and I ran it dry, ran lots of sand through it, and was generally very abusive to it.
3) They are kind of loud. Mine were even loud before I was abusive to them when they were new. But I had that tank in the back of the house where I didn't care if it was a little noisy.
4) The plastic they make the main body of the filter out of is kind of brittle. I mannaged to crack one when I don't think it should have cracked.
5) Impellars are a bit more expensive than I think they should be to replace, but that's just me being cheap.
6) The tabs on the motor that makes it attach to the body of the filter are weak due to aquaclear not having a radius on the underside edge. They break off too easily when installing/uninstalling the motor for service, and then you have to get a new motor or risk the motor falling off and dumping a ton of water on your floor.
Overall, I think if you can get them for $50-60, they're about as good as you can do for that price. For much more, I'd just order a canister (Eheim 2217, Mag 350, etc...).
But I'll always have an AC110 handy in my bucket of extra fish stuff. It's great for if I need to throw a filter full of carbon on a tank, swap bio media to cycle a tank, or if a filter dies I can dump all of the bio media in my AC110 to keep it alive until my other filter is up and running. They're very versatile, and I like the fact that I have it in case I need it.