Heres the review of the Viliv X70 EX with CarKit. The model I tested was the top of the line model with 32 Gb SSD and 3G Modem. The X70 is probably the best UMPC Ive used in the car. The car kit is very nice and having a built in GPS makes it very convienent. The 7" screen does make it easier to use and more usable while driving. Overall, I still prefer the S5 for its ability to easily fit in your pocket when you get out of the car, but if portability is not a concern or having a 7" screen is, you cant go wrong with the X70 and it does have a SD card slot which I do miss on the S5.
Dynamism (http://www.dynamism.com) has the X70 on sale now for $599.
For a more in depth review of the x70 check out http://www.umpcportal.com/2009/06/viliv-x70-ex-umpc-full-review-with-videos