SPV C500 here, a nice stablemate for the hx4700.
As to why I have both, well, I'm a bloke and they're really nice gadgets
The SPV is has the advantage of being small, fairly light and phone shaped (although what that means these days is open to debate). It's reliable and always handy. There's plenty of software around these days, and quite a bit that has both iPaq and SPV editions, and with Active Sync

the data can (mostly) keep in sync.
The iPaq is very good, but it's not always convenient to carry around, I see it more as a mini-PC.
As for the thread, we really need a single definitive point for a forum. iPaqHQ would be V good.