Well, I didn't read the books until around the time the fourth or fifth movie came out and I couldn't help but picture all the characters in 'muggle clothes' at all times. Except for the professors, I pictured them in what they wore in the movies. I guess I just have a bad imagination!
I started reading the book after watching the movies... but I loved the look half-Muggle/half-wizard as seen in the movies. Each attire reflected the status and the personality of each character. I think the costume designers in the movies captured that very well.
I've found Voldemort and Draco particularly alluring in Muggle clothes (Draco in HBP and DH - Voldemort at King's Cross, in OotP).
Severus is one of my favorite "men in black".
How could we imagine Dumbledore in something else than his 100% wizard's robes ? It was funny to see him in Muggle attire at the orphanage to meet young Tom Riddle.
Minerva is just great and mighty the way she is dressed. Curiously, for a Gryffindor, she enjoys wearing green garments.
Umbridge : perfect ! She's in pink but her attitude belies that look, it means she is not to be trusted at all. She may be a little girl deep down but she is not in her deeds.
Loved the look given to Sirius, Tonks and Moody.
The general feel of the movies costumes is that of a world that sticks to its traditions while being more or less influenced by the Muggle world around. I enjoyed that delicate balance.