The PowerBook 1400 has three real distinctions. First, it is widely considered to have one of the best keyboards that Apple has ever shipped on a portable. Second, it has a "BookCover" panel that allows one to place inserts underneath the plastic to change the look of the computer. Those are either transparent or black. Third, it is the first portable Apple computer to ship with an internal CD-ROM.
My thoughts on the keyboard are that it has a very nice feel, though the left side of the space bar doesn't push about one in ten times. This is just wear and tear. Other than that, the keys have a distinct click and you can just tell when the keys engage properly. You don't have to look at what you are typing to know when it typed correctly. The keyboard fits very well. My only complaint is that the metal-action keys are just a bit loud.
The BookCover panel is really cool. I had a crack in my top case that was beginning to stretch across the entire display housing, so I bought a replacement part from Wegener Media. Much to my surprise and delight, the case came with a only mildly worn clear panel, a part that I had been trying to track down for quite some time previously. The first thing I did was break out the Illustrator template and insert a piece of art behind it. In this case, it was from the Metroid universe. You can see it here. It looks like it was meant to be there. I've got a black panel if I need to be a little bit less conspicuous.
The CD-ROM is handy, but I'm on my second one of the 8x variety and it's becoming a bit finicky. It will make a disturbing clicking noise and refuse to spin up unless I push on the tray. The CD door snaps off easily under pressure. I have a replacement, but it is for the 12x model and I don't use it that often anyway. Strangely, audio CDs don't have a problem after they start playing, even homemade ones. I stick to floppies if possible. [editme] I'll set up a CD page for dealing with it later.
The rest of the computer is remarkably easy to open and upgrade. [editme] Upgrade section coming soon. I've modified and/or replaced the display housing, hard drive, CD-ROM, reseated the RAM, and added a wireless card plus probably at least a few abnormal software tricks. Those will probably go into the Pre-OS X directory.
My machine, at least, is a quirky little beast. Because I could go on about this for a while, see Quirks.