There have been quite a few different Walker style mowers on the market over the years. Obviously Walker is biggest name for this style but Exmark also still has one available (I believe). Several years ago Bob-Cat, Scag and Toro all made this style.
I did have a demo of the Bob-cat version and couldn't stand it. The thing clogged regularly and was slow compared to my ztr (which had a bagger on it too). So I would definately stay away from any used bob-cat version.
I hear good things about the Walker models but I have yet to actually try one out.
walker is probably the way to go if you have to bag in my opinion. Bagging is definately more time consuming, but as far as a good all around bagger I like the walker as good as any, as much as anything because they are good people to deal with. We have problems yes, and it will clog on occassion, but over all its a well built machine. You wont beat the cut they will give you, and you also have the option to change decks, etc.
If you bag everything, get a Walker or Exmark Navigator. They're both "the best", its just they each have a couple different features over the other. If you don't bag everything, you will need a side discharge deck for the Walker (not an option on Navigator). If you bag less then half the time, or switch back from discharging to bagging multiple times in the week, you will probably be more happy with a bagging system mounted on midmount zero turn. I like Grasshopper, just not terribly fond of of the cloth catcher top and the not terribly durable bags. Next midmount is probably gonna be red... Toro/Exmark has an extremely good catcher setup. Then again I may go to only bagging with the Walker and replace the midmount with a stand on or WB.