I know this has been mentioned before, but I thought I'd give it it's own thread and review.
I just bought the Cross Executive Wacom pen from Amazon for ~$40 + shipping:
http://www.amazon.com/Wacom-Cross-Ex.../dp/B0002FHEWA
Shipping was very quick, I ordered it on Friday, and received it on Tuesday. One of my primary tasks on the iLiad is taking notes throughout the day at work. I always thought the included wacom pen was too small, and got annoying and caused hand cramps if I wrote for any length of time.
Switching to the Cross pen has been a night and day difference. When you write with the iLiad stylus, you can feel the tip dragging on the screen. It feels a lot like writing with a pencil. I've always avoided pencils because of the feel of writing with one, and was a bit disappointed when I had that on the reason. The Cross pen, however, feels smoother and better than any pen I've used. Instead of "dragging" on the screen, it glides. I can't describe how nice it feels
I have the community version of the screen calibration and multi button patch installed on my iLiad (a version of this should be released with 2.11). Because the screen does not support tilt data, it takes a little bit to get the calibration just right. It's all about the angle you hold the pen when you're writing normally, and have to think of that when calibrating the screen. However, once it's calibrated, it's spot on when I write. The multi button patch also supports the other two buttons on the pen. Right now, no iRex apps have functionality for this built in, so they just see it as another click. But, I did test it with xournal (an app I ported). I was able to program the button on the back of the pen to erase when I flipped the pen over and used it like an eraser. I could also program functions for the click button on the pen itself. Once iRex adds this functionality to their note and/or ipdf apps, it will be very useful.
I should also note that the touch screen is not turned on when the factory pen is inside the iLiad. So you will need to pull it out slightly to use your Cross pen. It doesn't cause too many problems, but I can see it being an issue for people who move around and hold their iLiad when taking notes.
Also, the tip of the pen does look fragile (I haven't really tested it's durability yet), but it is replaceable and comes with 3 spare tips. A very nice addition.
Overall, I'm very happy with my purchase, and would recommend it to anyone who takes notes with their iLiad.