I just picked up the CX-9 with a nav package today and have used the nav a few times today. I also have a Garmin Nuvi 660 that I've used on travels and have a permanent powered cradle for it mounted in my wife's car (Mazda3 GT which in 2004 in Canada, didn't offer a nav). I've played with the CX-9 before deciding on the purchase and read somewhat negative reviews on Edmunds, so I wasn't expecting a great system. But while it's not absolutely great, after playing with it today, I actually think it's pretty good. The main missing feature is text-to-speech (which Nuvis 3xx and above have), but I haven't actually run into any built-in nav system that had that. Everything else is pretty good by comparison to the Nuvi.
Nuvi does have a few advantages. Text-to-speech, portability, traffic receiver if you live in the right place, SD card MP3 player ... but it is disadvantaged vs. a built-in system by not being built-in -- for effective use, requires a permanent wired in cradle which requires car modification of some form or another (trust me, I have EVERY mount available for the Nuvi, best setups require permanent wiring, etc.). By being portable, it also encourages car breakins and theft, which is a bigger deal than one even imagines (consider that your car probably has your garage door opener and your GPS has your home location ... get the idea: grab the GPS and the opener and voila, instant navigation AND access to your house). Also, some features on the CX-9 system are better than Nuvi (multiple intra-trip way points, keep out areas), but I've read that TomToms has those features covered.
I love the Nuvi, but I think in the CX-9, the built-in system wins on a balance of values. The lack of text-to-speech isn't quite as bad as I thought it would be and the CX-9 nav isn't anywhere as bad as I've read.
Oh, and the backup camera is quite good. I thought a lack of the backup ultrasonic sensors would be a major issue and was planning to purchase something aftermarket for the CX-9, but so far, the backup camera works well for the same purpose. Also, with the built-in nav system, you get your Bluetooth phone integrated with steering wheel controls, nav voice, and the sound system sound -- you won't get that nice integration with the Nuvi (but you will get Bluetooth with the Nuvi and it will silence it's own nav voice and it's own mp3 playback for phone calls, so perhaps this is a little moot).
Summary: Hard decision, but I am happy with my choice of the built-in system so far.
Cheers,
Lexx