We have 3 of them.  They are very handy.  The cows will follow one of these with fresh feed across the entire farm. 

One is a very narrow Farmco that isn't easy to chop into on a day with any wind but the cows can reach all the feed easily and it's easy to clean out the feed they don't eat.  It's 3 feet wide and long enough to feed 20 to 25 cows.  The second is a Pequa about 6 feet wide and good for chopping.  Clean out isn't easy.  It will hold four 5x5 round bales but you can spend a day cleaning out any hay they don't eat.  It feeds 25 to 30 cows at a time.  The third feeder is a flat hay wagon with steel slant bar oenings that will hold two round bales or a lot of green chop.  It's too high for calves and too wide for the cows to reach the center but real easy to clean.   The first two have dolly wheels in the front so they turn sharp.

Consider how easy they are to climb in and out to clean.  They should have a way to chain the tounge up out of the mud and so the cows don't bend it when they rub on it.  Don't get one too narrow for a round bale if you feed round bales. Some people like head locks so you can lock up the cows and give shots.  Headlocks probably double the cost.