I have several timers and like them all, depending on what I'm doing.
CED 7000: I use it for teaching and my personal training. The display shows the things I'm interested in (1st shot, split, shots fired and total). It has nice accessories that make it more user friendly (like the belt clip that swivels up, so I don't have to remove it to see my times). I also have separate battery back-up for it. It's nice and small, but the down side is that it's a little fragile, so I try to use one of my pocket pro's when running classes where it might get scratched or bumped. I keep this one in a case in my range bag.
Pocket Pro I/II: They're pretty much the same to me. The II has some different display settings, so it would be the only reason I might choose it over the I. They're pretty rugged compared to my CED 7000 and I treat them that way. They're usually in my target/gear box.
iPhone: I only use it for dry-fire as I don't like the idea of my phone getting "roughed" up on the range and an incoming call messes things up.