I definitely was much happier when I switched from a trunk pack to a pannier. The trunk pack I had was from Nashbar and velcroed on. It didn't leave room for the reflectors and stuff I wanted to put on there, plus it was such a pain that I never took it off, so I had to pack/unpack/repack every time.
The panniers I just lift off in 3 seconds and walk inside. I pack up the next day's stuff the night before, and in the morning it's just clip/hook/secure velcro and go.
I haven't mounted my trunk pack once since getting the panniers.