I really like it.
The idea is original, other version of the famous "Keep clam and carry on" but in Doctor Who, the most famous british TV show. I love the detail of the TARDIS and the color blue is perfect (just THIS blue, not other).
Other people overdo this poster and I don't like it. The originl idea is simple and the author treated her with respect.
I like too the phrase... Come along is perfect to this case. The Doctor always go with somebody but he is alone in the universe. It's so dramatic.
And finally, the result is totally "Don't worry, be happy", like the Doctor.