I knew I was gonna get this question asked.. The 7998 is a great deck.. I love it, but there's some things it can't do that the 9855 can..
1. Steering Wheel Interface - I have a brand new car with steering wheel controls and can't use them with the 7998. I've seen the IR type transmitters and heard negative things (such as lag, periodic non-response, etc) but the 9855 has an interface built in, and I really miss the steering wheel controls.
2. No iPod control - I've been looking for a way to get all my MP3s in the car. I'm so sick of the CD format, and I have 3 changers in my car right now. I bought an Omnifi already, but have now reconsidered since I would prefer to control an iPod with the 9855 instead of installing another faceplate on my dash.
3. Sirius compatibility - The 7998 was designed for XM, and the 9855 is setup for both, and I prefer Sirius over XM.
4. Lighting - The lights on the 7998 is only adjustable from green to amber, where as the 9855 has 512 choices. The amber on the 7998 doesn't quite match my dash...
Now I know these reasons may seem petty to some of you, but to me it's a matter of preference. I don't see the 9855 as being a big step backward. I know there are mixed reviews over the glidetouch, but I have used it and don't seem to mind it at all. And as far as what's available on the market, the 9855 is the closest match to my 7998 and the features I am looking for.
I hope this answers everybody's questions.