Mine has been breaking around in my wallet area b/c of all the mods . If you see cracks or paint chipping take it to a local welder, gussets are not hard to put in.
I bought my z in Aug of last year and it had less than 5 hours on it. I rode it a couple times then bought the Arens frame brace from a guy on Z400Central. I bottomed it out hard a couple times before I put the brace on. I also added Works Shocks not long after the brace and I'm sure that has helped out quite a bit but I still have no signs of stress on the shock mounts and it has not been rode easy
2003 Z400 Pro Circuit T4, K&N, Burgard +2's, LSR Axle, and Works Tripple Rates with Rezzies, and many more
2002 250EX Graydon Nerfs, K&N, AC Grab Bar, Pro Design Kill Switch
2001 Raptor 660R-For Sale, Yoshi full pipe, LSR axle, K&N, Jetted, warranty until 3-05
1985 ATC200X