I've posted about this before. I'm actually a product of both. When I went as a student visa, I did it myself but when I had to apply for PR, I attempted to do it myself and gave up on it due to the complexity of things. Initially I was apprehensive of using an agent due to the typical stereotype that they are a rip off. My view about them changed when I found a very reasonable agent - only charged $1,000. This was over 10 years ago...fast forward, I found myself in this profession and from the time I started my studies in immigration law, I promised myself I won't be one of those pricey ones especially for Filipinos as I know how it's like. I just had an expo over the weekend here in London as I'm based over here. most of my clients are Brits but of course, I do get Filipinos enquiring about Australia.
Anyway, engaging one is like hiring a tax agent. You can do it without. It's a personal choice but often times I've had clients who would come to me because they've messed things up and they ended up with more expenses and time wasted. If you are to do it yourself, be prepared to really do a thorough research. Especially these days when policies are much more fickle than before. However, if you are to engage someone make sure to check that they are registered. Ask for the MARN. The first 2 digits of their registration is an indication of when they first got registered. For example, mine starts with 11 which means 2011. So the older the year, the more experienced the agent is. Check out this link I posted about tips on choosing an agent http://pinoyau.info/discussion/10256/helpful-tips-on-choosing-an-agent#latest
Good luck!