I ALMOST ALWAYS book on CO-OL for the 1000 mile bonus. It has worked for me just fine. I've gotten the choice of Elite Seating, and gotten my 50% mileage bonus every time.
It IS slow, and on my recent flight where I upgraded with miles, my itinerary on CO-OL does not match what I've confirmed with a CO rep.
I use other services also.
Travelocity and ITN to verify my CO fare is good.
ITN to check F seats for upgrades.
I'll sometimes use ITA when I want to check alternate airports and fares. It would have saved me $300 (trip ended up cancelled) by using ALB instead of BDL and flying into SJC instead of SFO or OAK.
I've course, I'm also probably not the "power user" that many of you folks are!
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"I will not be pushed, filed, stamped, indexed, briefed, debriefed or numbered. My life is my own."