I purchased my DK45 in 2008 (new, but I think it was built in 2006). Starting off in the right gears (I think med range, 2nd gear) with the right throttle setting (2,000 RPM +/-) it would shudder something terrific!! I (certainly) noticed this in the dealer's lot well before negotiating a purchase price.
If I had a choice between 2 identical tractors, one with a smooth clutch, & one that chattered, sure I would have chosen the smooth operating one, but I had been tractor shopping for quite a while, and this was the only one I found that had everything I was looking for, without added things I didn't want, and in just the size / horsepower I was looking for; so I was willing to 'work around' the chattering clutch. I have driven several vehicles with chattering clutches, and have found I can (& do) develop 'techniques' to avoid (or minimize) the shudder.
The tractor had been setting on the lot for 2 years without selling (very likely because of the clutch), so the dealer seemed quite 'motivated' to sell; and I feel I got a very good deal. He never represented that the chatter would disappear after break-in (& I would have laughed at him if he did)
I have owned it now for 10 years & have 1,000 hrs on it. The chatter has now all but disappeared, I no longer 'baby' it, but if I try to make it shudder (just to see if it is still there!) , I have no problem "accomplishing the mission"! The clutch SLOWLY got better over maybe 300 / 400 hrs; certainly well past 'break-in' (& warranty).
I have never regretted buying it; but I certainly understand everyone's intolerance towards chattering clutches!
We have no real way of subjectively comparing my tractors "shudder" to your friend's tractor, so we may well be comparing apples to oranges.
My only real advice would be BEFORE you purchase, operate the tractor long enough to see what it would take to avoid the chatter; then decide if you are willing to do that for the rest of the time you own it. Do not believe any salesman that says the chatter will go away after break in!!!