This is a copy of a posting I made to the Zaurus developer boards. Since the question keeps coming up and I don't want to repost it all the time, I've put it on the web. Please don't consider this as a full review...
Klaus Weidner - firstname.lastname@example.org
I own both a C700 and a SL-5000D. A disclaimer - I haven't seen a 5600 yet, but from what I've seen on the web and here, it's essentially the same as the older models except for better battery life and an xscale processor.
In my opinion, it's no contest - get the C700. It has a keyboard that's actually useable for text entry. I've noticed that there's a minimum speed/efficiency needed to be able to take notes or write longer texts without going crazy with frustration - for me, Graffiti and the 5x00 mini keyboard are too slow, and the keyboard of the C700 is fast enough, as was my old HP 200LX. The full keyboard includes Ctrl, Alt, numbers available without contortions, and all important special characters for programming and function keys, and allows easy thumb typing.
The screen is amazingly good. Once you've seen it and used it for a while, you won't be able to go back to anything smaller. Just try covering three quarters of your computer monitor with paper (or switch it to 1/4 of the resolution you're currently using), and try to live with that for a while. 320x240 is IMHO useless for real work, since you can't even fit a readable 80x24 text window on it, and reading PDF or PostScript documents is extremely frustrating.
One other issue is RAM - the 5600 and C700 have the same memory configuration with only 32MB of RAM, and 64 MB flash. I don't understand why Sharp did it that way, 32MB isn't very much, and the 64MB flash are fairly useless since you'll need an SD/MMC storage card anyway if you want to store any serious amounts of data. I could imagine that some people may prefer a 5500 to take advantage of the 64MB RAM, but there's essentially no difference betwenn the 5600 and C700 in this respect.
The xscale isn't appreciably faster than the 206MHz StrongARM of the older models, so you shouldn't expect to get any speed improvements over the older models. It's not just a matter of recompiling applications - they have to be carefully tuned to take efficient advantage of the cache memory available, since all memory accesses are extremely slow compared to the clock speed. Since most applications will be memory-bandwidth limited, the xscale can't take advantage of the higher clock, and may even be slower in some cases since the StrongARM has a slightly faster memory bus (103 MHz vs 100 MHz). You'll need to wait for a new Zaurus that uses the 200MHz memory bus available in the new xscale version if you want to see any speed increase.
The battery life issue could be a factor depending on how you're using the PDA - you may need to get a backup battery for the C700 if you're on the road much. Keep in mind that the charger is tiny (about 50mm x 50mm x 16mm with the plug pins folded in), and you'll usually have an opportunity to recharge it during the day if you're using it constantly. If you're just using it as a normal PDA, the C700s battery life is enough to easily last the day.
I can't really comment on the software issues since I'm running Debian on my C700 (I don't like the Qtopia environments), but you may have some trouble with the japanese environment if you stick with the default software.
Support may be a more important issue - since the C700 is only available as an import, it'll be expensive or troublesome if something breaks.
So in summary, I can't think of any good technical reason to prefer the 5600. If you need to save money, get a (used?) 5500 instead, otherwise go for the C700.