Now this is coming from a football player not a sprinter so take it for what its worth, but IMO Plyo's have helped me a great deal, i am currently 6'4 290 and was stuck at 4.9 in the 40 for quite some time, even with a consistent and sound speed program i was pretty stuck at around this time. My coach suggested that I try to add a depth jump routine into training (i had done prep work and felt prepared, however was a little worried that my body wouldnt be able to handle it as was the case in the past) however after a 4 week training cycle with depth jumps (along with my speed routine) i retimed and ran a 4.72 on grass and a 4.68 on a track. i was also able to dunk for the first time and I tested my power clean from the hang at the end of the cycle and it also jumped up about 25 lbs.
During the routine I did change some things around to allow for proper recovery, and for me personally I would not do a training cycle with them in for more then 4-5 wks just becasue they are such a high intensity excersice (when they are out of my routine I will usually incorporate them 1 day every 3-4 wks just as a jolt to my system). My foot contacts for a heavey session are between 40-50 and for a light session 10-20, with rest up 3-4 min between sets and volume in a set not exceeding 5. when doing the 1 day every 3-4 weeks i usually will keep the foot contact between 20-30.
I have and do do other plyos but just wanted to share my experiences with depth jumps (also i felt no breakdown in my body like i was worried about, i adapted really well, i think in large part because i was smart with the volume and didnt get too aggresive with them too soon)
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