medea: i am still waiting for my writing score before posting a full write-up of the experience, but I definitely will.
In short though, if you are scoring in the 300-500 range, it is more likely you do not have a good grasp of the fundamentals. And in my humble opinion, I think you should try to focus on that instead of use up all the practice questions.
You may not think I am qualified to make that assumption/suggestion since:
... preview to my write-up ...
* I have an undergraduate degree in Biomedical Engineering from a U.S. private college ranked in the top 50 (strong quantitative background).
* I have lived in the U.S. for the past 10+ years, so decent grasp on the English language.
* I scored 720 on the GMATPrep 1 only after doing the practice/familiarization questions included in the GMATPrep software (which I still think was a fluke).
Even then, when I go back to review the verbal fundamentals, I realized I missed quite a few things. I might have known those things subconsciously due to the long term exposure to the language, but it is still nice to have a firm concept in mind. After that, understanding the test strategies and figuring out things to look for just became so much easier since I have a firm foundation instead of rely on my intuition, which is often wrong in the case of SC.