I haven't played Zen either, but do have Pinball FX which is made by the same people. It was OK when it was on sale and is a fun diversion, but nowhere near as good as the Williams collection. It feels at times too arcadey on certain tables. However, if you love pinball, it's probably still a good deal. There are some free tables as well as some premium ones, including a new Excalibur one released this week. I just snagged the free Buccaneer table and bought the Street Fighter table when I got the game originally.
The advantage it does have over the Williams collection is the option for online multiplayer.
"I assure you, whatever the others promise to do, when it comes to the showdown, they won't be there."