I've had talks with folks from some of the bigger bid managment companies, and taken a look at a couple of the smaller guys too. I came away with the thought that there is not yet a "best" tool. Each of them has a core competency like datafeed managment, or paid inclusion managment or Google managment. But a major drawback that I've found with all of the smaller solutions and even some big name companies is scalability; some of them would work great for 50-100 keywords, but adding more keywords either breaks the system or adds ridiculous costs for simple 'actions' like checking the bid.
Folks like Efficient Frontiers or Channel Advisor seem to work well for big companies, but smaller guys might not like the hefty fees. I've heard glowing testimonials though, so maybe it's worth a shot.