Mcaveman,
Yes, I purchased the rear bagger for my z121s, 48". The cost was right at $1k, installed. I am not entirely happy about the purchase. When I got it home I stared using it to collect grass/leaves to use as mulch in my garden & compost fodder in my compost pile. It collects just fine, but it does reach capacity pretty quick. The only other rear bagger I have used was on my craftsman where I can reach behind me to feel how full a bag is - - that is not possible on the Kubota. The bags are too far away. Second, the bags themselves are pretty danged heavy before you fill them with debris. Once filled, I find them hard to deal with. I will admit I am not as strong as I was a few years [or decades] ago. My solution has been to make very short runs so I don't fill the bags entirely.
I also have a complaint about the z121s throwing out grass from the front of the deck in front of the discharge chute. It does this with/without the rear bagger. I had hoped the bagger would help this problem, but no.
OK time to take the bagger off so I am not slowed down by having to stop so often & empty bags - - Kubota does have what looks to be a neat system that uses 3 round knobs that you supposedly are to loosen in order to take off the bagger chute. The problem was/is those knobs are locking [not sure of the appropriate term] - - the closer the knob is to snugging up, the more difficult it is to turn. I could just turn 2 of the knobs, but one was down lower where it was difficult to grip except with the tips of my fingers & thumb. No way could I turn with any force - - had to resort to some sliplocks. Once I got the knobs off, I went straight to the hardware store to get the right size nuts & lock washers to take the place of the knobs. Now I can attach/unattach using a deep socket. Much easier.
OK, now that I have removed the bagger chute it is time to replace the normal discharge chute. First of all, you have to remove one of the threaded posts that are used to hold the bagger chute in place, or the discharge chute will not fit. Not hard, just another step.
I had forgotten how difficult the task was on my craftsman to replace the discharge chute so that the chute is kept down by that little spring. In fact, on the Kubota, I've started using the mower with the chute held down only by gravity- - - seems to work well enough just leaving the spring off.
Overall, I like the z121s with the exception of its tendency to blow grass/leaves out the front right of the deck in front of the discharge chute. But, I am not real fond of the rear bagger.
Hope this makes sense.
Jack
. Now I have to go by Barlows and check out a mulch kit for my little Z.:thumbsup: If it costs me thousands of dollars and get another new hat(s) I will never take your advise again, well not if it require visiting Barlows. 
