I often switch ringtones since I can't really find one good one to stick to. Mostly have custom ringtones for people in my phone book.
Only phone that I can't openly switch ringtones is my SO705i (sony ericsson ntt docomo model) which uses MMF files @ (MA-7 coding). They have the idea of anti-piracy in Japan so all MP3s are locked under DRM and you can't use your own MP3 to make into ringtones. They need to be converted to MMF (SMAF) so it just becomes tedious that I just use whatever they give me. :(