The VNAV on the 737 is not very good or stable. From a high altitude descent, you can often get descent rates initially of 4,500fpm, especially when it thinks it is too high or having trouble maintaining the computed profile, even if you have entered various flight level winds to help it calculate more accurately. It common to have to manually take over in LChng or VS when the speed is rapidly approaching barbers pole, and it will breach speed limits unless you do take over. It seems very unstable sometimes, not at all like the 747 system which is identical, but works well. ROD varies a lot, but generally reduces lower down to 2000fpm+. One has to carefully check what speed/altitude parameters are input on STARs or set required ones yourself as this affects the computed profile, which is displayed as a sort of GS needle on the PFD. The descent calculation is unreliable sometimes, and it can suddenly recompute and come up with a different profile. Descending into Leeds is especially bad for some reason, the descent can suddenly change by several thousand feet. Then if you get any directs, it recomputes the whole thing again. Compared to the 747, it is not very good.