I was initially put off by the list price of the 6300 when it was announced, but they're already being sold for well below list and with the rebate/free-ink offer it was just too good of a deal to pass up.
So far I'm very happy. I had previously been printing with an ipf5000, so some of the features from the x100 series are new to me, such as calibration, better handling of media type settings, etc. The new custom media type support is nice, although there's room for improvement - custom types can be exported from one printer and loaded onto another, but only for the same model; 6300 and 8300 can't share custom media types, they have to be manually created for each model.
The Photoshop Print Plugin now has the ability to use Adobe's CMM, which means you can do rel-col with BPC in the plug-in now; no need to do "convert to profile" in Photoshop first. The plugin also has some proofing options that may be useful for some, but it's not something I have a need for. The accounting module in the driver/status-monitor is another nice edition. It lets you track materials cost on a per-job basis - not just ink but also paper. Unit costs can be entered by the user, even for custom media types.
I know some felt that the first-generation print heads weren't lasting as long as they should have, but I believe Canon made improvements with the x100 series' print heads. The x300's get yet another revision, which will hopefully last as long as or longer than the 2nd-generation heads. I'm a fairly low-volume printer, so it's not something I'm terribly concerned about, to be honest.