Ultimately the decision comes to how much you value your money. It's quite simple from how I see it. First, choose if you want to do a prep course. All these people who did prep courses saying that they were beaten by their friends who didn't do them are irrelevant really. We all know the UMAT is a test mainly of natural ability, so it's irrelevant comparing against others. Instead, the question they should've been answering is whether they improved after having done them. I'm pretty sure the answer to that should be yes, at least for section 3 and in my opinion, also section 1. I agree with the consensus that section 2 is unable to be studied, and even if you do go to a prep course, don't even waste your time trying to study it. For those who said studying didn't help them whatsoever, then obviously they've been using the material wrong. I'm not saying these people are idiots since it's easy to fall into that trap. The UMAT is a very difficult test to study for since it's quite conceptual in the way you need to approach it. Practice involves altering one's own thought processes and most people aren't familiar with this. I get the feeling most people study it like they do with VCE trial exams and try and just read solutions, taking in the actual steps done rather than the thought processes involved in knowing what steps to do. The solutions will tell you the steps involved in the question, but it's up to you to think about the solution to getting to the solutions - this is where most will struggle.
Next however is that you have to realise that although prep courses are going to make you improve, the question of how much it can is one that's difficult to answer. As I said above, I'm quite certain that section 1 and 3 can be improved through study and the difference mainly comes down to your own skill at studying as I've described above. Next though, here's where you have to think about how much the UMAT (and its sequelae, i.e. Med) actually means to you. When you're talking about using the UMAT to get into Med, every point matters really. You're going to be ranked on it so from my perspective at least, an extra few percentile is something I'd definitely pay for. In my case however, I'm pretty sure it was more than just a few percentile anyway having scored 100%ile in section 3 - a feat I doubt I could've gotten anywhere near if I hadn't prepared.
Lastly is well, which prep course to choose. Well, out of MedEntry and NIE, I'm undecided. Haven't had much experience with NIE other than a few hours with their books. I felt NIE wasn't challenging enough and didn't mimic the structure of the real UMAT very well, but neither does MedEntry in that regard. MedEntry is probably overly difficult in fact. The hardcopies given by NIE are a big bonus though. Still, those who have done both are probably in a better place to comment.