Thought I'd add mine into the mix as well:
GRE Powerprep #1: 162V, 155Q
GRE Paper #1: 162V, 157Q
Manhattan Diagnostic: 156V, 160Q
Manhattan prep test: 147V, 157Q
GRE Paper #2: 160V, 157Q
Actual GRE results: 167V, 160Q, 4.5 AW
Study period was over about 3 months, about 6-8hrs/week of two 3-4hr blocks per week (with maybe 5-10 extra hours of during the week leading up to the test). The 5 practice tests I took were more or less evenly spaced over those 3 months, with one a week before the actual
From my experience (and based on my score), the manhattan diagnostic and ETS practice tests seemed more or less accurate for quantitative, but verbal seemed wildly different during the actual test (practice tests seemed much more dependent on vocabulary, while the actual test felt more like an emphasis on reading comp than anything else).
As other posters have noted, Manhattan testing and materials are almost completely focused on vocabulary building, which is no longer helpful as the new GRE format focuses on reading comprehension and less on those ridiculously obscure and arcane analogies.
