Ok, this is from my own point of view.
I used a 5L for a batch of brown ale and it sealed and worked pretty good. There was an excessive amount of carb when I opened it, but laying it back on it's side and releasing the gas before popping the top worked to fix that. The beer was good.
As I understand it, you can get 5-7 batches average out of them before they start to rust. They are difficult to clean.
Since I now have a kegerator, I simply stopped using the 5Ls and plan on filling up growlers from the keg using BierMunchers awesum beer gun. But, you could also bottle directly into growlers if you dont' have a kegerator.
The drawback is not having a cool little cooling machien to keep the thing cold, but I honestly can't see spending that much money when an Ice Bucket and Ice cost so little.
So, personally I think that even though they work ok, I'd not do it again. There are alternatives. And, if you are thinking about using the Heineken Kegs, I think they are difficult to use because they have something inside.
A plain 5L mini can be bought at some brew supply stores, along with the plastic bungs for the top.