Where predictive tests can go wrong is when you consult your astrologist, numerologist, eChing, Human Design or even my own test (called Genetic Energetics), and make a decision about the relationship based wholly on the outcome of the test or reading. Even worse, you don't commit to the actual test results, but look for what you predicted would show up. In this way, you become your own worst enemy.
Even with the huge bias I have for my own compatibility test, I know that the most important thing it does is provide people with a far more illuminated awareness of their partners' or prospective partners' unique characteristics. This creates an opportunity to talk about each other openly and ask better questions about each others' needs.
So if you are going to take a test, get your head on straight and look for the most enlightening information you can get about who the other person is, how they express themselves to you and how willing they are to see you for who you really are. Avoid anything that claims to tell you whether you're compatible if there's no scientific proof behind it. If you don't, you'll likely be drawing conclusions, making decisions and setting expectations that are completely inaccurate for you. This causes far more harm than good.
If you're ready to get married or commit to a long-term relationship, I absolutely recommend that you take a completely different kind of test than the experiential tests or the predictive compatibility tests. There are a number of great sites that will give you anywhere from a few dozen to a couple hundred questions to answer candidly with your partner.
These are questions that help you find out if you're on the same page and truly ready to enter a longterm committed partnership or marriage. They cover topics about religion, politics, sleeping habits, your home, eating, vacations and hobbies, pets, your upbringing, child-rearing, sex, money and much more. These are the must-have discussions that every couple should go through together before they make any long-term commitments. Not doing so is simply immature and irresponsible.
Here's the bottom line: use the experiential tests to take your time and experience what it's like to be with your partner in circumstances that are important to your happiness. Be aware that chemistry will likely blind you in some way. So take your time.
If you want to take a test like the MatchMatrix Energetic Compatibility Test, know that this information is there so you can be aware of your core similarities and differences and make enlightened decisions about your ability to be present and loving. And most importantly, go online, to your church or your local bookstore and get some extensive questionnaires to go through with your partner. Be real. Be honest and be a good listener. The value can last you a lifetime or save you from a mismatch that should never have happened. Here are some great questionnaires to put to use: FOCCUS Pre-Marriage Inventory, Prepare/Enrich: Building Strong Marriages and Relate Institute.