I used them for about a year before I gave up on them and upgraded to a ball-lock keg system.
Of course at the time they served their purpose and performed quite well. As far as cleaning the minis, I would use gobs of oxyclean or straight-A with superhot water, fill up the keg and let it sit for at least a half hour. Then I would rinse them with very hot water several times, I would fill it up with hot water shake and empty it over and over until it did not smell like anything.
I clearly remember the mini-keg that I filled prior to buying a ball-lock system. It wasn't rusted however I cleaned it what I had thought was sufficient enough...but I could still smell a little something in it...you know that stale beer aroma. So I ingorned the aroma (DWRHAHB), filled it, primed it, put it in a dark corner to carbonate and came back to it two weeks later. Unveiling the mini-keg displayed what appeared to be a huge freakin lump coming out of one of the sides.
As I tried to tap it (it was obviously over-carbed) beer just started shooting EVERYWHERE. And you would think it would stop after a while. oh no. not so much.
So, my advice is because minikegs can be a pain to clean thoroughly, they lack a bunghole big enough to get a big scrubber in there....upgrade to a ball lock or pin lock system. Save your mini-kegs for when you want to bring a portion of a batch to someone's house or something. Yeah I know you're probably like "dude, I want to use my little keg!" and I'm like yeah man, use them....but you will see just as a lot of us have that switching to a 5-gallon system will make things a lot easier in the long run.
I can speak from experience, I've still got 2 tap-a-draft taps, 4 PET bottles and 3 mini-kegs. I still use them when I'm testing small batches or when I know I am taking part of a batch to someone's house etc. The TAD is awesome for that.
The other question you can ask yourself is, do you feel like putting the beer that you've spent your hard earned money and time brewing into a rusty mini-keg to even RISK something going wrong with it? DUde, that's up to you man. But I wouldn't do it. It's not worth the time and pain. If you got something rusted just throw the darn thing away. It's not worth the risk. It's the equivalent of cooking with some bacon that "smelled kinda funny but should be OK". Do you really want to find yourself sitting on the toilet ***tting like a mink?
I didn't think so.