Like you I got NX free with the D300. I tried it, I think the results are excellent but I cannot live with the speed of it. I will use it again in the future, but probably only on single images that I want to spend time editing for the best quality. It would take days to go through a large set of pics with.
I've used Capture One 3.x as my main RAW processing tool for the last year. It's quick and simple to use and produces great results. There's still no sign of v3.x for the D300 so I gave 4.0 a go yesterday. So far results look good, it's like lightning in comparison with NX. I'm not a fan of the interface yet, but like anything new it takes some time to familiarise. It took a while to work out how to turn the sharpening by default down to a much more reasonable level. The default sharpening was way OTT for my tastes.
I ran some quick comparisons on output with C1, Lightroom and ACR. There wasn't a lot to choose between them. I didn't have NX installed on this new machine to test, but I'd imagine it will be slightly better.
There's a version of Bibble out that supports D300 NEF files as well now. I've not used that before but may give it a go. I think these may well all be stop-gaps before C1 release a compatible version of 3.x though. It's what I'm used to