I haven't connected from the CarPC but I regularly connect from my Powerbook or Palm Tungsten|T.
You can connect in either of two ways; the phone can be a regular modem and allow you to use a dial-in ISP or using GPRS through your cell phone company network. I use both ways since GPRS is not available everywhere.
T-Mobile just uncapped its GPRS service so there is no data limit on downloads! In comparison other companies bill $40 for 1 or 2MB of data.
Over GPRS speed is ok, Certainly not cable modem or ISDN but maybe just a tad faster than 56k. Using dial-in speed is noticeably less than 56k. For either method I use RSS feeds or the mobile version of websites for browsing.
Definitely worth it. I can't wait to do it from the CarPC.