I rented a Befco Seeder last year and they DO work. They're very fast (I did 2 acres in 45 minutes) but you do have to run two passes at 90 degrees from one another or else your grass comes in like you're planting corn!
I don't think they're worth the money though, unless you do this for a living. This year, I rented a power walk-behind rake and did my yard with that, then used my First Choice Core Aereator (high quality unit, much less cost than many others) and then shot the seed out with a hand-pushed rotary spreader. The grass came in just as good - if not better - and my rental costs were reduced to just one-quarter the price of renting the Befco unit.
The real trick with overseeding is that you MUST have the seed come into contact with bare dirt, and you MUST have peatmoss or straw over the top of the new seed to shade it from the sun. Direct sunlight kills the young seeds (which many do not know). It's the heat that makes the germination, and the frequent watering, not sunlight. So most the work is in the soil prep and the cover after you drop the seed.
Unfortunately, I've become something of an exprert in lawn turf renovation....<hate it>
-DUane C