From what I've read, it seems that public transport is an affordable, easy way to travel in the city areas. Transperth is the public transport service in the Perth metropolitan, and using their SmartRider card gets you discounts on trains, buses, and ferries. They also have free CAT (Central Area Transit) buses for travelling within the city.
Outside the city, though, it looks like driving is the best option, especially if you're looking to go to Swan Valley, or Margaret River. There are wineries and views that you'd miss if you took public transport, but it would probably get you there quicker? Any opinions on this?
I'm more of a public transport person myself. I find that not having to drive means I get more time to look out of the window and appreciate the scenery. There are downsides, of course — you have to stay on the designated route, and wandering off can be a bit of a hassle.
Great Northern Highway south-eat of Fitzroy Crossing. Image by Peter Boer, Wikimedia Commons.
Perth Airport, like every other airport, has car rental and hire. I've never really rented a car before, so I'm not sure what the usual prices are, but WebJet has a page where you can compare prices from different companies.
As you mentioned, although public transport is frequent and reliable in most suburbs, if you want to travel further out of the city services aren't so regular and it can get expensive. By the way, the second photo that you used for this article is not of Perth, Australia - it's of the Errochty Forest in the UK.
It's definately easier to use public transport in the city (parking is no easy task & if you do find a space, it will cost you a small fortune to park there). But it is essential to have a car for travelling around out the city, in my opinion. There are lots of lovely places to visit in & around Perth, you will not be disappointed!