I use the IPAC for the game controller I built for MAME, it is truly awesome. Everyone else is always talking about 'hagstrom this' and 'hagstrom that' (referring to the 800lb gorilla of keyboard encoders), but the i-pac is a better value, better technologically, and works nicely.
MAME = Multiple Arcarde Machine Emulator. Its an arcade game emulator that plays over 4000 actual arcade games. It plays the actual arcade versions of games as opposed to the home gaming system ports. You can get it here: Mame. Hagstrom is another keyboard encoder that used to be real popular amonst people building their own arcade cabines for Mame. Their KE-72 encoder is still considered to be the grand daddy of keyboard encoders because it offers 72 inputs. I use one in my arcade cabinet as a matter of fact. The IPAC has become the encoder of choice because its a better value. It also offers some functions the hagstrom's don't. I'm using an IPAC in the jukebox i'm building because the hagstrom is way overkill for the project. Hagstroms are good encoders, but unless you actually need more inputs than the IPAC offers, an IPAC is the way to go as far as keyboard encoders go. You can find more info on the hagstroms here: Hagstrom Electronics
Well the x-keys appear to be a little diferent than the typical keyboard encoder. First off from what their website says, the x-keys uses a matrix for the keys. What this means is that there are not dedicated inputs. For instance, the x-keys supports 96 inputs. However there are only 26 header pins. By using various combinations of the pins by column and row you get 96 inputs. This can make it a hassle to wire. It also can lead to ghosting(keypress of keys that weren't actually pressed). Now one thing nice about the x-keys is that it you program macros into it. This means that you could wire a button to perform several keystrokes. For example ctrl - c(windows cut). Not sure how usefull this would be, but its something the x-keys has that the IPAC doesn't. Hagstroms offer this as well. FYI for anyone looking at the hagstrom's, the reason the models with less inputs are so expensive is because they can be hooked to matrixed keypads. This is different than the x-keys which uses a matrix for its inputs. The hagstrom ke-18, ke-24, and LP-24 allow you to hook a matrixed keypad up without modification. As reference when i hooked a matrixed keypad up to my IPAC, i first had to figure out how the matrix was setup and had to cut traces and make new connections so that each button could have a dedicated signal and ground. This took me around 5 hours to do. I'm no electronics whiz however. It's my understanding that you would not have to do that if you're using one of the mentioned hagstrom encoders. I could be wrong however. So basically unless you need the macro function, i would go with an IPAC over an X-keys. But thats just my personal opinion.
I agree also. I have two IPACs in MAME machines and I love them both. Hagstroms too expensive.
As for the X-keys unit, I don't think that ghosting is really and issue here unless you're pressing massive keys all at once. Sure, it's based on a matrix design, which is horrible for an arcade/gaming controller, but I don't see it being abused like a controller for a car setup.
I still say IPAC, however, especially since it's cheaper and I know just how easy it is to use and set up.