I have a wireless Bell computer on my handle bars so I can check my current speed. The odometer function lets me keep track of total miles on the bike. I use Endomondo on my smartphone for total workout distance, calories burned, average pace, etc. It's nice having all my rides in a database on their web site so I can see my statistics from week to week. I just start the application, press the button to turn off and lock the screen, and toss it in my handle bar bag and forget about it. I carry a plastic zip lock bag in case I run into rain.
Exactly what I do, but I use it in "beat your own time mode" and put in a place where I can here the pace announements. When it says "1km in 1:30 seconds" it always make me smile. When it says "1km in 3:xx seconds" then its time to ride harder. I also run a basic cateye enduro 8 (wired) so I can monitor time, distance, speed and it gives me a pace arrow to motivate me to ride harder. Combined with my smart phone running endomondo I get all the info I need. I agree that battery life is sad, but thats easy enough to counter with a few extra batteries stashed in the Jersey pocket.