I used to hate these ETS tests but recently I've come around and started to appreciate them.
1) I think studying for ETS tests especially the subject tests can be extremely useful. Unlike some of the questions your teachers give, the questions on ETS tests are usually perfectly worded and have an unambiguous answer. So you can only blame yourself if you do poorly. I think the questions are really fair and well-thought out to make them thought-provoking yet not really painful since they are designed to be answer in about a minute.
2) I think it is a reasonably accurate assessment of your knowledge and understanding of physics. It straightforwardly tests how well you know physics and that is why it is required.
Anyway GO1, my point is that you shouldn't feel like the time you spend studying for the Physics GRE is wasted. I have actually been enjoyed a lot studying for the math GRE (which I am probably taking in October also). It has shown me what my strong points are and what topics I need to work (stupid auto-link) on. The problems are pretty fun and are at a good level for me. So, you can learn a lot from GRE preparation if you view it more as a regular course than as just a chore or a graduate-school "rite-of-passage".
EDIT: oh yeah, good luck; since you have even helped me and lots of others with physics homework I'm sure you will do well