I agree that sometimes it works and that sometimes you have to wonder, "Why??" A remake gone ugly is The Fog. The original (1980) with Adrienne Barbeau is genuinely scary and has a great cast. When the remake came out, I was SO excited....until I saw it.
A remake I did love was Dave, which I thought was a remake of Moon Over Parador (awesome, awesome movie). As it turns out, they both are based on The Prisoner of Zenda. Both of those movies had amazing casts and were incredibly funny.