It's not the smallest thing to tote around, but it's doable. I've used mine in various rental cars.

The setup is a bit of a pain. You'll want to probably suction cup something to the windshield. If you're alone, using the car kit's included mount and mounting to your left is probably fine. I like having mine to the right, so I bought a windshield mount with a long arm attached so I could mount it just above the dash. Either way, you'll still have to fish a wire to the power socket, and you'll have to fish the antenna out to the roof through one of the doors. I haven't had a problem just closing one of the car doors over the antenna cable. I fish it out the passenger side or one of the back doors. 5 minutes or so of setup is probably enough once you're used to it. You want a unit with a built-in FM transmitter (which I think they all have) as many of your rentals won't have the aux in jack. (Take that cable in case they do, though, as it will save the pain of finding a clear FM frequency to use. Although I do the 87.9 thing which works almost everywhere.)

If you're going to use it in your own car (if you have one), you'll make life at home a lot easier if you get an additional car kit so you have one for your own car and one for on the road with rentals. That way in your own car you can more permanently arrange the thing and not have to fish the antenna wire and get everything set each time. Then what you want is one of the Sirius receivers that truly plugs into a dock, as opposed to plugging in with wires. Looks like most of the new units do this now, though, with the new universal dock system. So the rest is just a choice of looks, display size (and thus unit size) and a few extra added features. The main cost of the service is in the subscription anyway, not really the radio.