I see two reasons to want a CB radio in your Interstate. First, if you participate in WBCCI caravans, they use CB channel 14 for intra-caravan communication. Second, when you are driving down the highway and see traffic coming to a stop ahead, it's handy to put the CB on channel 19 and listen to the trucker chatter to find out what's going on. Other than that, I leave mine turned off.
I can tell you my own experience. My CB radio is a Cobra 29 LT "Classic" which, as the name implies, has been around roughly forever. It has a good reputation and can be had for less than $100. One advantage of the 29 is that it has a built-in SWR bridge so you can do a quick check on your antenna and feedline without any external equipment.
The antenna is the most important part of the system, really. If you are willing to do it, the best position for the antenna is smack dab in the middle of the roof, so the roof serves as a ground plane.
I cheated; I use a K-40 3 foot Superflex fiberglass whip (normal-mode helix in case there are any antenna engineers out there) mounted in the middle of the fiberglass cap on our Chevy K-1500. I constructed an artificial ground plane on the inside of the cap using sticky-back copper tape, soldered together at intersections and bonded to the rest of the truck at the ends.
This combination has worked very well; I once worked 20 miles mobile-to-mobile with another caravanner on channel 14 in the very sparsely populated west. In more congested areas--like around Chicago--you won't be able to work more than a mile simply because there are so many stations using the channel.
The antenna installation does need to be set up by somebody who knows what they are doing and preferably has a separate instrument known as an "antenna analyzer".
Personally, I wouldn't waste my time with a hand held unit; the tiny "rubber ducky" antennas are practically worthless at 27 MHz and you'll be lucky to get a quarter mile range, as cwf said.