Depending on how they're programmed, they cruise between .48-.77 Mach (316 - 508mph at 40,000 feet). They can be stopped by conventional antiaircraft missiles and artillery, but they can come in at low levels, below the radar detection threshold - which means that in general you either get lucky and someone eyeballs them first who has the ability to do something about it, or the first you hear about them is when they arrive at their destination. The disadvantage is that they take hours to get anywhere and if some sharp patrol aircraft (hello AWACS) sees them, there is usually time to shoot them down, and they're so slow that a guy with a rifle can bring them down if they're on the deck. To prevent this sort of thing, stealth technology is being applied to cruise missiles to reduce the chances of them getting spotted.
ICBMs (and other purely ballistic missiles) on the other hand, can carry a heavier payload and have a lower flight time (10-20 minutes). The disadvantage is that there is no surprise when you launch an ICBM - the moment it lifts off, it's clearly obvious what it is and that it's been launched (thanks to the dozens of ballistic launch detection sats in orbit) as well as where it came from. Until recently, it didn't matter, because there was no known proven way to shoot down an ICBM and the only "solution" was MAD. That's not the case any more. There is no such thing as a stealth ICBM. (There are stealth MIRV units, but that's another story.)