Don't own one, but I know of a few people who do.
Now, I'm a little biased against them but...
The C500 (SP3) was a decent phone when it came out because it was the first small and cheap Windows Smartphone and it used Windows Smartphone 2003, which was a BIG step up from the 2002 edition (which crashed a lot).
However, the C500 has been on the market for around 6 months now (as long as your current 6230). People have started to notice that the build quality is poor (dust getting behind the screen, paint flaking off the keys, keys getting stuck, etc) and it's missing quite a few features compared to the latest phones.
I haven't had a play with the SP3i, but I've heard that it's just a slight revision of the SP3. Still no megapixel camera, still no 3G support, still no in-built GPRS counter, still no hot-swappable memory, etc.
If you want to do some more research for yourself then
MoDaCo and
MS Mobile News are good starting points.