I have to agree with ShyT about TCAS, it is a fantastic tool and once you have used it, you don't really want to fly without it.
It was critical at my last company as we would often be returning to base after 9 in the morning and would usually try and get descent OCA to try and speed things up for our return, rather than being left very high until the last minute causing us to have to scream down.
When you are doing over 200kts at around 3000' OCA, then you really don't see an awful lot of traffic, TCAS was utterly invaluable for helping us pick our way back in safely. If we didn't have it fitted (tight wad company!) then we would never risk taking an early descent even if it was gin clear with two crew keeping their eyes open. (We were supposedly IFR as well.........)