I use LogMeIn Ignition on the iPad to occasionally access both my Mac and my Windows machine at work. It is fast, works well, but...it really brings home (to me at least) how different the touch interface is from the mouse/keyboard interface...dragging that little mouse around with your finger then tapping it to click just gets old quickly
We use LogMeIn on all our PCs/Macs, so its mostly just familiarity as to the reason I went with it on the iPad.