I have been using iKey for years, from well before the daunting new UI. I have found it consistently to be bug-free and stable. Once you get your mind around the UI, the actual process for setting up shortcuts is not complicated. To the contrary, I have found it quite intuitive.
I use Quicksilver as much as iKey, and it is somewhat misleading to compare them. Quicksilver is primarily an app launcher and only minimally customizable. It indexes everything on your computer and gives you access to it, like Spotlight, but better (IMO). iKey is a shortcut program. It doesn't index your computer. You can set up shortcuts to launch applications, but you can create shortcuts for so much more. Actions that normally take several steps can all be set up to run automatically with one key-stroke shortcut. You can program delays between the different steps to give them time to execute. iKey's reach is amazing in this regard. Moreover, the latest release(s) now allow you to use more than one alphabetic key, which extends the number of shortcuts exponentially. I hate to sound like a promo, but what you can do in iKey is almost unlimited.
Butler is more like Quicksilver, an access/launcher program for indexed items. I have tried it several times and have found it less simple and elegant and intiutive than Quicksilver, but unless I am selling Quicksilver short, Butler gives you much more reach and customization. On the other hand, Quicksilver works with a key-stroke combination whereas Butler must be visible and open somewhere on your desktop, and I am a nut for the former.
Let me point out as well that Quicksilver tends to run slowly, at least in my experience. For a simple, clean app launcher, which is as near as instantaneous as one could get, try Namely. And for AddressBook info, try TapDex. This triples the number of apps and keyboard shortcuts you would use to access data, but it cuts down run time significantly.