They are not like Radiohead. Here's a newsflash:
No one is like Radiohead.
True, if you were to pick between Radiohead and Metallica, then Coldplay is more like Radiohead. But do not go into any of their albums expecting anything short of evolved Karma Police esque songs. Expecting stuff like Creep, Just, or Paranoid Android, and you'll be vastly disappointed.
For me, Coldplay is probably my favorite band of all time, at least right now. They have to fight with Rage Against the Machine, Foo Fighters, Matthew Good, and Muse for top billing, but they're there right now.
Their songs aren't identical, but they often re-sample little aspects of their music (listen to Speed of Sound and then Clocks, and you'll see what I'm talking about). Nevertheless, I really like the lead singer's voice, I really like the way the guitar sounds, and I just flat out like the way their songs come together post production (though they're really good live too). If you're looking for some of their songs to check out, my favorites are:
Yellow
Trouble
Talk
Speed of Sound
Beautiful World
Square One
In My Place
Clocks
What If
The Scientist
They're a direct mix between a classical/soft band and a rock band. None of their songs are close to hard rock, but you're not going to hear anything quite like Yellow or Talk in most soft music categories. Personally, "Trouble" is tied for my favorite song of all time (tied with Everlong).
Coldplay rocks, and their new album is pretty good. A lot of people are disappointed with it because they wanted more of an evolution from the band and their music, but in my books, there's no band that can match Coldplay's quality in their genre (Keane is good, but just good), and I'm perfectly fine with getting new music that falls under the same category, since it's just so good.
That's my opinion of Coldplay, anyway.